feat: split the heavy site pages

A manufacturer page over 500 KB of Markdown becomes a summary: its
largest consoles move to pages of their own, cut into parts when one
console is still too large, and every console keeps its heading there.
MAME's 1,564 cards move to four pages, its page keeping a row per entry
with the card's old anchor. RetroDECK's arcade list gets its own page.
site.js follows a moved card by content or by its row.

The largest page drops from 10.5 MB to 0.88 MB. Emulator groups of the
navigation open by initial (65 KB to 41 KB before the content). File
cards are level 3 under a level 2, and the hook keeps the table of
contents of card pages to level 2: platform pages no longer list every
system id. Key/value cards name their columns. The sweep runs before
the decoration, which kept every page no generator wrote any more.

scripts/generate_site.py

@@ -1430,6 +1430,8 @@ def _browser_title(relative: Path, title: str) -> str:
     }
     if key in index_titles:
         return index_titles[key]
+    if key in PART_PAGES:
+        return title
     if key.startswith("emulators/"):
         return f"{title} emulator firmware"
     if key.startswith("systems/"):
@@ -1474,6 +1476,9 @@ def _page_description(markdown: str, relative: Path, title: str) -> str:
     }
     if key in specific:
         return specific[key]
+    if key in PART_PAGES:
+        return (f"{title}: every file with its hashes, size, source references "
+                "and who reads it.")
     if key.startswith("platforms/") and relative.stem != "index":
         return (f"{title}: where the BIOS files go, the command that installs them, the pack, "
                 "and every file by system with its hashes.")
@@ -1510,9 +1515,13 @@ def _page_description(markdown: str, relative: Path, title: str) -> str:
 # Structured data a page generator knows beyond its text, by page path
 # relative to docs/: read by decorate_markdown_pages.
 PAGE_DATA: dict[str, dict] = {}
+# Pages that hold part of a page too long for one (a console of a split
+# manufacturer, a run of an emulator's cards), by path relative to docs/:
+# their H1 already says what they are.
+PART_PAGES: set[str] = set()
 
 
-SYSTEM_PARTS_IN_FULL = 1000
+SYSTEM_PARTS_IN_FULL = 250
 
 
 def collection_dat(db: dict, generated_at: str) -> str:
@@ -1601,8 +1610,9 @@ def system_structured_data(slug: str, consoles: dict[str, list]) -> dict:
                 part["citation"] = citations
             parts.append(part)
     if len(parts) > SYSTEM_PARTS_IN_FULL:
-        # game-engines would carry 2.6 MB of JSON-LD: past the cap a page
-        # lists its catalogue-verified files and points at the full table.
+        # A page of cards carried up to 2.6 MB of JSON-LD (game-engines), a
+        # quarter of its weight: past the cap a page lists its
+        # catalogue-verified files and points at the full table.
         return {
             "hasPart": [part for part in parts if part.get("citation")],
             "isBasedOn": urllib.parse.urljoin(SITE_URL, "downloads/files.csv"),
@@ -1824,6 +1834,11 @@ def emulator_structured_data(profile: dict) -> dict:
         if members:
             part["hasPart"] = members
         parts.append(part)
+    if len(parts) > SYSTEM_PARTS_IN_FULL:
+        # MAME's 1,564 entries and their members were 614 KB of JSON-LD: past
+        # the cap the members are left to the profile's JSON.
+        for part in parts:
+            part.pop("hasPart", None)
     if parts:
         data["hasPart"] = parts
     return data
@@ -2044,14 +2059,23 @@ def generate_platform_index(coverages: dict, registry: dict | None = None) -> st
     return "\n".join(lines) + "\n"
 
 
+# A platform system whose file list passes this much Markdown gets a page
+# of its own: RetroDECK's arcade list was 750 KB of a 1.1 MB page.
+PLATFORM_SECTION_BUDGET = 200_000
+
+
 def _render_platform_systems(
-    by_system: dict, config_files: dict, emulator_files: dict
+    by_system: dict, config_files: dict, emulator_files: dict,
+    platform: tuple[str, str] | None = None, moved: dict[str, str] | None = None,
 ) -> list[str]:
     """One collapsible block per system, listing every file it needs.
 
     Split out of generate_platform_page: the block is most of that
     function's branching, since each file renders a different set of
     hashes, sizes and provenance depending on what the platform declares.
+    With ``platform`` (its key and name) and ``moved``, a list longer than
+    PLATFORM_SECTION_BUDGET goes to a page of its own, written into
+    ``moved`` by stem, and the system's heading stays with a link to it.
     """
     lines: list[str] = []
     # Per-system detail sections (collapsible for large platforms)
@@ -2154,6 +2178,26 @@ def _render_platform_systems(
             if f.get("category_evidence"):
                 lines.append(f"{pad}    {_category_evidence_md(f['category_evidence'])}")
 
+        block = lines[block_start:]
+        if moved is not None and platform and sum(len(line) + 1 for line in block) > PLATFORM_SECTION_BUDGET:
+            key, display = platform
+            stem = f"{key}-{anchor}"
+            body = block[2:] if use_collapsible else block
+            moved[stem] = "\n".join([
+                f"# {display}: {sys_id}", "",
+                f"Part of the [{display} page]({key}.md#{anchor}), whose other systems "
+                f"are listed there. Each file below is as {display} declares it, with "
+                "its status in the pack.",
+                "", "## Files", "",
+                *search_excluded(
+                    [line[len(pad):] if line.startswith(pad) else line for line in body]),
+                "", f"*Generated on {_timestamp()}*",
+            ]) + "\n"
+            lines[block_start:] = [
+                f"{_count(total, 'file')}, {ok_count} verified: listed on "
+                f"[a page of their own]({stem}.md).", "",
+            ]
+            continue
         # The section's heading stays searchable, its file list with every
         # hash is the lookup's: retrodeck's arcade list was 495 KB of index.
         lines[block_start:] = search_excluded(lines[block_start:])
@@ -2386,9 +2430,13 @@ def generate_platform_page(
     cov: dict,
     registry: dict | None = None,
     emulator_files: dict | None = None,
+    moved: dict[str, str] | None = None,
 ) -> str:
     """A guide first (folder, command, pack, check), then every file the
-    platform's list and its emulators need, under a reference heading."""
+    platform's list and its emulators need, under a reference heading.
+
+    ``moved`` collects the pages of systems whose list is too long for it.
+    """
     registry = registry or {}
     config = cov["config"]
     display = cov["platform"]
@@ -2485,7 +2533,8 @@ def generate_platform_page(
             "</div>",
             "",
         ]
-    lines.extend([*band, "| | |", "|---|---|"])
+    lines.extend([*band, '<div class="rb-meta-card" markdown>', "",
+                  "| Detail | Value |", "|---|---|"])
     if hash_type:
         lines.append(f"| Hash type | {hash_type} |")
     if version:
@@ -2519,6 +2568,7 @@ def generate_platform_page(
                 f"| Contributed by | [@{username}](https://github.com/{username}):"
                 f" {contribution}{pr_link} |"
             )
+    lines.extend(["", "</div>"])
     if name not in GUIDES:
         detect = (registry.get(name, {}).get("install") or {}).get("detect") or []
         on_card = any(rule.get("method") == "card_marker" for rule in detect)
@@ -2576,7 +2626,8 @@ def generate_platform_page(
     lines.append("")
 
     lines.extend(
-        _render_platform_systems(by_system, config_files, emulator_files)
+        _render_platform_systems(by_system, config_files, emulator_files,
+                                 (name, display), moved)
     )
 
     lines.append(f"*Generated on {_timestamp()}*")
@@ -2701,8 +2752,8 @@ def file_location_items(
         parts = entry.get("path", "").split("/")
         if len(parts) < 3 or parts[0] != "bios":
             continue
-        slug = parts[1].lower().replace(" ", "-")
-        items.append({"sha1": sha1, "url": f"systems/{slug}/#sha1-{sha1[:12]}"})
+        page = card_page(entry.get("path", ""), sha1)
+        items.append({"sha1": sha1, "url": f"systems/{page}/#sha1-{sha1[:12]}"})
     # A data directory's files are listed on the page of an emulator reading
     # the tree: 2,534 hashes the packs ship were found nowhere on the site.
     owners = _data_member_owners(profiles or {})
@@ -2772,7 +2823,7 @@ def file_lookup_records(
             continue
         entries = profile.get("files") or []
         for entry, anchor in zip(entries, emulator_file_anchors(entries)):
-            url = f"emulators/{key}/#{anchor}"
+            url = f"emulators/{emulator_card_page(key, anchor)}/#{anchor}"
             members = [(str(entry.get("name", "")), entry)] + [
                 (str(member["name"]), member) for member in entry.get("contents") or []
                 if isinstance(member, dict) and member.get("name")
@@ -3207,8 +3258,9 @@ def _emulator_links(sha1: str, consumers: FileConsumers) -> list[str]:
         index = entries.get(emulator)
         anchor = (_emulator_card_anchor(emulator, files_of[emulator], index)
                   if index is not None and emulator in files_of else "")
+        page = emulator_card_page(emulator, anchor) if anchor else emulator
         fragment = f"#{anchor}" if anchor else ""
-        links.append(f"[{emulator}](../emulators/{emulator}.md{fragment})")
+        links.append(f"[{emulator}](../emulators/{page}.md{fragment})")
     return links
 
 
@@ -3379,14 +3431,16 @@ def _card_title(text: str, anchor: str, searchable: bool = True) -> str:
     """A file card's heading, so the site search lands on the card.
 
     The search splits a page at its headings: a card titled in bold sent a
-    pasted md5 to the top of a page of a hundred cards. Level 4 stays out
-    of the table of contents (toc_depth 3), and a card without an anchor
-    keeps the id the toc gives it.
+    pasted md5 to the top of a page of a hundred cards. Level 3, under
+    the console or the Files heading: a level 4 after a level 2 skipped
+    one for screen readers on 551 pages. The table of contents of a page
+    of cards stops above it (mkdocs_hooks.py), and a card without an
+    anchor keeps the id the toc gives it.
     """
     attributes = f"#{anchor} .rb-file-title" if anchor else ".rb-file-title"
     if not searchable:
         attributes += " data-search-exclude"
-    return f"#### {text} {{ {attributes} }}"
+    return f"### {text} {{ {attributes} }}"
 
 
 def system_install_choices(coverages: dict) -> dict[str, dict[str, list[tuple[str, str]]]]:
@@ -3418,6 +3472,197 @@ def system_install_choices(coverages: dict) -> dict[str, dict[str, list[tuple[st
     return choices
 
 
+# A manufacturer page above this much Markdown becomes a summary with one
+# page per console, and a console above it is cut into parts. 500 KB of
+# Markdown renders to about 750 KB of HTML: game-engines was one 10.5 MB
+# page that took 21 s of main-thread work on a phone.
+SYSTEM_PAGE_BUDGET = 500_000
+
+# A system card: whether it is a variant, its lines, its sha1.
+Card = tuple[bool, list[str], str]
+
+
+def _console_cards(
+    manufacturer: str,
+    files: list[dict],
+    consumers: FileConsumers,
+    anchored: set[str],
+) -> list[Card]:
+    """Each card of one console, primaries by name then variants, with
+    whether it is a variant."""
+    searchable = manufacturer not in GAME_DATA_TOPS
+    main_files = [f for f in files if "/.variants/" not in f["path"]]
+    variant_files = [f for f in files if "/.variants/" in f["path"]]
+    cards = [(False, _system_file_card(f, consumers, anchored, searchable), str(f.get("sha1", "")))
+             for f in sorted(main_files, key=lambda x: x["name"])]
+    cards += [(True, _system_file_card(v, consumers, anchored, searchable), str(v.get("sha1", "")))
+              for v in sorted(variant_files, key=lambda x: (x["name"], x["path"]))]
+    return cards
+
+
+def _cards_markdown(cards: list[Card]) -> list[str]:
+    """Primaries, then the variants in a closed block."""
+    lines = [line for variant, card, _sha1 in cards if not variant for line in card]
+    variants = [card for variant, card, _sha1 in cards if variant]
+    if variants:
+        # A variant gets the same card as a primary: the revision a
+        # platform pins is the one a reader comes to find, and showed as
+        # a name and a truncated MD5, nothing said who pins it.
+        lines.append(f'??? note "Variants ({len(variants)})"')
+        lines.append("")
+        for card in variants:
+            lines.extend(f"    {line}" if line else "" for line in card)
+        lines.append("")
+    return lines
+
+
+def _parts(cards: list[Card], budget: int) -> list[list[Card]]:
+    """Cards in runs of at most ``budget`` bytes of Markdown, in order."""
+    parts: list[list[Card]] = [[]]
+    size = 0
+    for card in cards:
+        weight = sum(len(line) + 1 for line in card[1])
+        if parts[-1] and size + weight > budget:
+            parts.append([])
+            size = 0
+        parts[-1].append(card)
+        size += weight
+    return parts
+
+
+_NEXT_FROM_SYSTEMS = [
+    "[Get your files](../which-pack.md), for the setup you play on",
+    "[When a game does not start](../wiki/troubleshooting.md)",
+    "[Every emulator](../emulators/index.md), and the files each one reads",
+]
+
+
+def system_pages(
+    manufacturer: str,
+    consoles: dict[str, list],
+    consumers: FileConsumers,
+    installs: dict[str, list[tuple[str, str]]] | None = None,
+    budget: int = SYSTEM_PAGE_BUDGET,
+) -> dict[str, tuple[str, dict[str, list]]]:
+    """A manufacturer's pages by stem under systems/: one page, or a summary
+    and a page per console when the whole would pass ``budget``. Each page
+    comes with the files whose cards it holds, by console.
+
+    Every console keeps its heading on the summary, so a link to
+    systems/<maker>/#<console> still lands, and site.js sends a card
+    anchor the page no longer holds to the card's new place. The pages
+    each card and console went to are recorded for every other link
+    (siterender.record_console_pages).
+    """
+    slug = _slug(manufacturer)
+    json_link = (f"[JSON](../api/v1/systems/{urllib.parse.quote(slug)}.json)")
+    whole_anchors: set[str] = set()
+    sections = {
+        console: _console_cards(manufacturer, consoles[console], consumers, whole_anchors)
+        for console in sorted(consoles)
+    }
+    lines = [
+        f"# {manufacturer}", "",
+        f"The files below by console, with every hash, are also published as {json_link}.",
+        "",
+    ]
+    for console, cards in sections.items():
+        lines.append(f"## {system_icon_markdown(manufacturer, console)}{console}")
+        lines.append("")
+        lines.extend(_console_install_block(console, (installs or {}).get(console, [])))
+        lines.extend(_cards_markdown(cards))
+        lines.append("")
+    lines.extend(_next_block(_NEXT_FROM_SYSTEMS))
+    lines.append(f'<div class="rb-timestamp">Generated on {_timestamp()}.</div>')
+    whole = "\n".join(lines) + "\n"
+    if len(whole) <= budget:
+        return {slug: (whole, consoles)}
+
+    # The heaviest sections move to pages of their own until what stays on
+    # the manufacturer's page is half the budget: a section of one file
+    # keeps its place, and a reader does not open 91 pages to browse Other.
+    weights = {
+        console: sum(len(line) + 1 for _variant, card, _sha1 in cards for line in card)
+        for console, cards in sections.items()
+    }
+    moved: set[str] = set()
+    for console in sorted(weights, key=lambda c: (-weights[c], c)):
+        if sum(w for c, w in weights.items() if c not in moved) <= budget // 2:
+            break
+        moved.add(console)
+
+    pages: dict[str, tuple[str, dict[str, list]]] = {}
+    summary = [
+        f"# {manufacturer}", "",
+        f"The files below by console, with every hash, are also published as {json_link}. "
+        f"The largest sections have a page of their own, linked from their heading.",
+        "",
+    ]
+    summary_anchors: set[str] = set()
+    for console in sorted(consoles):
+        summary.append(f"## {system_icon_markdown(manufacturer, console)}{console}")
+        summary.append("")
+        if console not in moved:
+            summary.extend(_console_install_block(console, (installs or {}).get(console, [])))
+            summary.extend(_cards_markdown(
+                _console_cards(manufacturer, consoles[console], consumers, summary_anchors)))
+            summary.append("")
+            continue
+        # Cards are rendered again for their own page: the anchors a page
+        # holds are its own.
+        cards = _console_cards(manufacturer, consoles[console], consumers, set())
+        parts = _parts(cards, budget)
+        base = f"{slug}-{_slugify_anchor(console) or 'files'}"
+        stems = [base if index == 0 else f"{base}-{index + 1}" for index in range(len(parts))]
+        record_console_pages(slug, console, stems, [
+            [sha1 for _variant, _card, sha1 in part] for part in parts
+        ])
+        title = f"{manufacturer}: {console}"
+        count = _count(len(consoles[console]), "file")
+        if len(stems) == 1:
+            summary.append(f"{count}, on [their own page]({stems[0]}.md).")
+        else:
+            summary.append(f"{count}, on {len(stems)} pages by file name:")
+            summary.append("")
+            for index, (stem, part) in enumerate(zip(stems, parts)):
+                summary.append(f"- [Part {index + 1}]({stem}.md): {_part_range(part)}")
+        summary.append("")
+        for index, (stem, part) in enumerate(zip(stems, parts)):
+            heading = title if len(stems) == 1 else f"{title}, part {index + 1} of {len(stems)}"
+            lines = [
+                f"# {heading}", "",
+                f"Part of [{manufacturer}]({slug}.md), in [Systems](index.md). "
+                f"Every hash of these files is also published as {json_link}.",
+                "",
+            ]
+            if len(stems) > 1:
+                others = ", ".join(
+                    f"[part {i + 1}]({other}.md)" for i, other in enumerate(stems) if i != index)
+                lines.extend([f"This part holds {_part_range(part)}. The others: {others}.", ""])
+            lines.extend(_console_install_block(console, (installs or {}).get(console, [])))
+            lines.extend(["## Files", ""])
+            lines.extend(_cards_markdown(part))
+            lines.append("")
+            lines.extend(_next_block([f"[{manufacturer}]({slug}.md), every section"]
+                                     + _NEXT_FROM_SYSTEMS))
+            lines.append(f'<div class="rb-timestamp">Generated on {_timestamp()}.</div>')
+            held = {sha1 for _variant, _card, sha1 in part}
+            pages[stem] = ("\n".join(lines) + "\n", {console: [
+                f for f in consoles[console] if str(f.get("sha1", "")) in held]})
+    summary.extend(_next_block(_NEXT_FROM_SYSTEMS))
+    summary.append(f'<div class="rb-timestamp">Generated on {_timestamp()}.</div>')
+    kept = {console: files for console, files in consoles.items() if console not in moved}
+    return {slug: ("\n".join(summary) + "\n", kept), **pages}
+
+
+def _part_range(part: list[Card]) -> str:
+    """The first and last file names of a part, as the cards show them."""
+    names = [re.search(r"`([^`]*)`", card[2]).group(1) for _variant, card, _sha1 in part]
+    if len(names) == 1:
+        return f"`{names[0]}`"
+    return f"{_count(len(names), 'file')}, `{names[0]}` to `{names[-1]}`"
+
+
 def generate_system_page(
     manufacturer: str,
     consoles: dict[str, list],
@@ -3427,53 +3672,10 @@ def generate_system_page(
     """One manufacturer's page, a section per console.
 
     ``installs`` gives, per console, the (platform, system id) pairs whose
-    install command the section offers.
+    install command the section offers. A page over the budget is the
+    summary system_pages writes, its consoles on pages of their own.
     """
-    slug = manufacturer.lower().replace(" ", "-")
-    lines = [
-        f"# {manufacturer}",
-        "",
-        f"The files below by console, with every hash, are also published as "
-        f"[JSON](../api/v1/systems/{urllib.parse.quote(slug)}.json).",
-        "",
-    ]
-    anchored: set[str] = set()
-    searchable = manufacturer not in GAME_DATA_TOPS
-
-    for console_name in sorted(consoles.keys()):
-        files = consoles[console_name]
-        icon_md = system_icon_markdown(manufacturer, console_name)
-        lines.append(f"## {icon_md}{console_name}")
-        lines.append("")
-        lines.extend(_console_install_block(console_name, (installs or {}).get(console_name, [])))
-        main_files = [f for f in files if "/.variants/" not in f["path"]]
-        variant_files = [f for f in files if "/.variants/" in f["path"]]
-
-        for f in sorted(main_files, key=lambda x: x["name"]):
-            lines.extend(_system_file_card(f, consumers, anchored, searchable))
-
-        if variant_files:
-            # A variant gets the same card as a primary: the revision a
-            # platform pins is the one a reader comes to find, and showed as
-            # a name and a truncated MD5, nothing said who pins it.
-            lines.append(f'??? note "Variants ({len(variant_files)})"')
-            lines.append("")
-            for v in sorted(variant_files, key=lambda x: (x["name"], x["path"])):
-                lines.extend(
-                    f"    {line}" if line else ""
-                    for line in _system_file_card(v, consumers, anchored, searchable)
-                )
-            lines.append("")
-
-        lines.append("")
-
-    lines.extend(_next_block([
-        "[Get your files](../which-pack.md), for the setup you play on",
-        "[When a game does not start](../wiki/troubleshooting.md)",
-        "[Every emulator](../emulators/index.md), and the files each one reads",
-    ]))
-    lines.append(f'<div class="rb-timestamp">Generated on {_timestamp()}.</div>')
-    return "\n".join(lines) + "\n"
+    return system_pages(manufacturer, consoles, consumers, installs)[_slug(manufacturer)][0]
 
 
 # Emulator pages
@@ -3914,7 +4116,7 @@ def _render_emulator_file(
             # Which copy answers this entry, linked to its collection card
             # (provenance, every hash, download): an entry declaring no hash
             # said "in repo" without naming the file.
-            page = parts[1].lower().replace(" ", "-")
+            page = card_page(held, sha1)
             details.append(
                 f"Collection copy: [{_code(held)}]"
                 f"(../systems/{urllib.parse.quote(page)}.md#sha1-{sha1[:12]})"
@@ -3988,7 +4190,7 @@ def _held_copy_line(copy: tuple[str, str], consumers: FileConsumers) -> str:
         archive = _code(path)
         zsha1 = consumers.sha1_at(path)
         if zsha1 and len(parts) >= 3 and parts[0] == "bios":
-            page = urllib.parse.quote(parts[1].lower().replace(" ", "-"))
+            page = urllib.parse.quote(card_page(path, zsha1))
             archive = f"[{archive}](../systems/{page}.md#sha1-{zsha1[:12]})"
         return f"Collection copy: member {_code(member)} of {archive}"
     if parts[0] == "data" and len(parts) >= 2:
@@ -4020,8 +4222,7 @@ def _directory_copies(entry: dict, consumers: FileConsumers) -> list[str]:
     parts = folder.split("/")
     where = _code(folder + "/")
     if len(parts) >= 3 and parts[0] == "bios":
-        page = urllib.parse.quote(parts[1].lower().replace(" ", "-"))
-        where = f"[{where}](../systems/{page}.md#{_slugify_anchor(parts[2])})"
+        where = f"[{where}]({console_href(_slug(parts[1]), parts[2], '../')})"
     tree = f"{REPO_URL}/tree/main/{urllib.parse.quote(folder)}"
     return [f"Collection copies: {_count(len(members), 'file')} under {where} "
             f"([browse]({tree}))"]
@@ -4369,6 +4570,41 @@ _DATA_MEMBERS: dict[str, dict[str, dict]] = {}
 _DATA_REVISIONS: dict[str, dict] = {}
 
 
+# An emulator page lists at most this many file cards: MAME's 1,564 made a
+# 3 MB page. Past it, the cards go to pages of their own and the emulator's
+# page keeps a table of every entry, each row holding the card's old
+# anchor. A count, not a size: system cards link to these cards and are
+# written before them.
+EMULATOR_CARDS_PER_PAGE = 400
+
+
+def emulator_parts(name: str, files: list) -> list[tuple[str, range]]:
+    """The pages an emulator's cards go to, with the entries each holds,
+    or none when they fit on the emulator's page. Balanced, in profile order."""
+    if len(files) <= EMULATOR_CARDS_PER_PAGE:
+        return []
+    count = -(-len(files) // EMULATOR_CARDS_PER_PAGE)
+    size = -(-len(files) // count)
+    return [(f"{name}-files-{index + 1}", range(start, min(start + size, len(files))))
+            for index, start in enumerate(range(0, len(files), size))]
+
+
+def plan_emulator_pages(profiles: dict) -> None:
+    """Record which page holds each emulator card, before any page links one."""
+    stems = set()
+    for name, profile in sorted(profiles.items()):
+        if profile.get("type") in ("alias", "test"):
+            continue
+        files = profile.get("files") or []
+        anchors = emulator_file_anchors(files)
+        for stem, entries in emulator_parts(name, files):
+            stems.add(stem)
+            record_emulator_cards(name, stem, [anchors[i] for i in entries])
+    clash = stems & set(profiles)
+    if clash:
+        raise RuntimeError(f"an emulator's card page takes a profile's address: {sorted(clash)}")
+
+
 def generate_emulator_page(
     name: str,
     profile: dict,
@@ -4379,13 +4615,28 @@ def generate_emulator_page(
     data_dirs: list[dict] | None = None,
 ) -> str:
     """One emulator's page. ``data_dirs`` defaults to the profile's own."""
+    return emulator_pages(
+        name, profile, db, consumers, data_names, data_registry, data_dirs)[name]
+
+
+def emulator_pages(
+    name: str,
+    profile: dict,
+    db: dict,
+    consumers: FileConsumers | None = None,
+    data_names: set[str] | None = None,
+    data_registry: dict | None = None,
+    data_dirs: list[dict] | None = None,
+) -> dict[str, str]:
+    """An emulator's page by stem under emulators/, and the pages its cards
+    went to when there are more than EMULATOR_CARDS_PER_PAGE."""
     if profile.get("type") == "alias":
         parent = profile.get("alias_of", profile.get("bios_identical_to", "unknown"))
-        return (
+        return {name: (
             f"# {name}\n\n"
             f"This core uses the same firmware as **{parent}**.\n\n"
             f"See [{parent}]({parent}.md) for details.\n"
-        )
+        )}
 
     emu_name = profile.get("emulator", name)
     emu_type = profile.get("type", "unknown")
@@ -4419,12 +4670,13 @@ def generate_emulator_page(
     if data_dirs is None:
         data_dirs = profile.get("data_directories", [])
 
+    part_pages: dict[str, str] = {}
     lines = [
         f"# {emu_name}",
         "",
         '<div class="rb-meta-card" markdown>',
         "",
-        "| | |",
+        "| Detail | Value |",
         "|---|---|",
         f"| Type | {emu_type} |",
     ]
@@ -4522,16 +4774,25 @@ def generate_emulator_page(
     else:
         _file_available = _availability_check(db, data_names)
 
+        lines.extend(["## Files", ""])
         lines.extend(_emulator_file_summary(files, _file_available))
 
         anchors = emulator_file_anchors(files)
-        for f, anchor, alias in zip(files, anchors, emulator_file_aliases(files, anchors)):
-            lines.extend(
-                _render_emulator_file(
-                    f, profile, consumers, files, _file_available, anchor, name,
-                    alias,
-                )
-            )
+        aliases = emulator_file_aliases(files, anchors)
+        cards = [
+            _render_emulator_file(f, profile, consumers, files, _file_available, anchor, name, alias)
+            for f, anchor, alias in zip(files, anchors, aliases)
+        ]
+        parts = emulator_parts(name, files)
+        if not parts:
+            for card in cards:
+                lines.extend(card)
+        else:
+            lines.extend(_card_index(emu_name, files, anchors, aliases, parts))
+            for number, (stem, entries) in enumerate(parts, 1):
+                part_pages[stem] = _emulator_part_page(
+                    name, emu_name, number, parts, [cards[i] for i in entries],
+                    [files[i] for i in entries])
 
     if data_dirs:
         # The member tables are a file list with hashes: out of the search,
@@ -4593,6 +4854,63 @@ def generate_emulator_page(
         "[When a game does not start](../wiki/troubleshooting.md)",
     ]))
     lines.extend([f"*Generated on {_timestamp()}*"])
+    return {name: "\n".join(lines) + "\n", **part_pages}
+
+
+def _card_index(
+    emu_name: str, files: list[dict], anchors: list[str], aliases: list[str],
+    parts: list[tuple[str, range]],
+) -> list[str]:
+    """Every entry of a split emulator page, one row each, linked to its card.
+
+    Each row holds the ids the card had on this page, so a link cited
+    before the split lands on the row, and site.js follows it to the card.
+    """
+    page_of = {index: stem for stem, entries in parts for index in entries}
+    lines = [
+        f"{emu_name} reads more files than one page holds: each entry below "
+        f"links to its card, on {_count(len(parts), 'page')} by profile order "
+        + ", ".join(f"[{number}]({stem}.md)" for number, (stem, _e) in enumerate(parts, 1))
+        + ".",
+        "",
+        '<section class="rb-card-index" data-search-exclude markdown>',
+        "",
+        "| File | Required | Category |",
+        "|------|----------|----------|",
+    ]
+    for index, (f, anchor, alias) in enumerate(zip(files, anchors, aliases)):
+        ids = "".join(f'<span id="{a}"></span>' for a in (anchor, alias) if a)
+        shown = str(f.get("path") or f.get("name", ""))
+        lines.append(
+            f"| {ids}[{_code(shown)}]({page_of[index]}.md#{anchor}) "
+            f"| {'yes' if f.get('required') else 'no'} | {f.get('category', 'bios')} |"
+        )
+    lines.extend(["", "</section>", ""])
+    return lines
+
+
+def _emulator_part_page(
+    name: str, emu_name: str, number: int, parts: list[tuple[str, range]],
+    cards: list[list[str]], files: list[dict],
+) -> str:
+    """One page of a split emulator's cards."""
+    first = str(files[0].get("path") or files[0].get("name", ""))
+    last = str(files[-1].get("path") or files[-1].get("name", ""))
+    others = ", ".join(
+        f"[{n}]({stem}.md)" for n, (stem, _e) in enumerate(parts, 1) if n != number)
+    lines = [
+        f"# {emu_name} files, part {number} of {len(parts)}",
+        "",
+        f"Part of [{emu_name}]({name}.md), whose page has the profile, the install "
+        f"command and every entry. This part holds {_count(len(files), 'entry', 'entries')}, "
+        f"{_code(first)} to {_code(last)}, in the profile's order. The other parts: {others}.",
+        "",
+        "## Files",
+        "",
+        *(line for card in cards for line in card),
+    ]
+    lines.extend(_next_block([f"[{emu_name}]({name}.md), the profile and every entry"]))
+    lines.extend([f"*Generated on {_timestamp()}*"])
     return "\n".join(lines) + "\n"
 
 
@@ -5947,19 +6265,35 @@ def nav_emulators(profiles: dict) -> dict[str, dict]:
 
 
 def _emulator_nav(profiles: dict) -> list:
-    """Emulators in alphabetical groups ("A to C"), by the name a user knows."""
-    groups: dict[str, list[tuple[str, str]]] = {}
+    """Emulators in alphabetical groups ("A to C"), then by initial.
+
+    Material writes the open section of the navigation into every page: a
+    group of 96 emulators put 65 KB of links before each emulator page's
+    content. One initial is at most 60.
+    """
+    groups: dict[str, dict[str, list[tuple[str, str]]]] = {}
     for key, profile in nav_emulators(profiles).items():
         display = str(profile.get("emulator", key))
         initial = display[:1].upper()
         letters = next((g for g in EMULATOR_NAV_GROUPS if initial in g), "")
         label = f"{letters[0]} to {letters[-1]}" if letters else "0 to 9"
-        groups.setdefault(label, []).append((display, f"emulators/{key}.md"))
+        sub = initial if letters else label
+        groups.setdefault(label, {}).setdefault(sub, []).append((display, f"emulators/{key}.md"))
+
+    def entries(items: list[tuple[str, str]]) -> list[dict]:
+        return [{d: path} for d, path in sorted(items, key=lambda e: e[0].casefold())]
+
     order = ["0 to 9"] + [f"{g[0]} to {g[-1]}" for g in EMULATOR_NAV_GROUPS]
-    return ["emulators/index.md"] + [
-        {label: [{d: path} for d, path in sorted(groups[label], key=lambda e: e[0].casefold())]}
-        for label in order if label in groups
-    ]
+    nav: list = ["emulators/index.md"]
+    for label in order:
+        if label not in groups:
+            continue
+        subs = groups[label]
+        if len(subs) == 1:
+            nav.append({label: entries(next(iter(subs.values())))})
+        else:
+            nav.append({label: [{sub: entries(subs[sub])} for sub in sorted(subs)]})
+    return nav
 
 
 def _system_nav(manufacturers: dict) -> list:
@@ -6174,14 +6508,8 @@ def _generate(args: argparse.Namespace) -> None:
     # report the same files.
     gap_report = build_emulator_gap_report(profiles, coverages, db, suppl_names)
 
-    print("Generating static API and bulk data exports...")
-    exports = generate_data_exports(
-        docs, db, coverages, profiles, stats, gap_report
-    )
-    write_if_changed(str(docs / "data.md"), generate_data_page(exports) + REFERENCE_NEXT)
-    PAGE_DATA["data.md"] = dataset_structured_data(exports, db)
-
     # Build system_id -> manufacturer page map (needed by all generators)
+    plan_emulator_pages(profiles)
     print("Building system cross-reference map...")
     manufacturers = _group_by_manufacturer(db)
     set_system_page_map(
@@ -6189,25 +6517,6 @@ def _generate(args: argparse.Namespace) -> None:
     )
     print(f"  {len(_system_page_map)} system IDs mapped to pages")
 
-    export_reports = load_export_reports()
-    _EXPORTED.update(report["platform"] for report in export_reports)
-
-    print("Generating platform pages...")
-    write_if_changed(
-        str(docs / "platforms" / "index.md"),
-        generate_platform_index(coverages, registry),
-    )
-    for name, cov in coverages.items():
-        # The emulators this platform runs, not every one of the system.
-        runs = resolve_platform_cores(cov["config"], profiles)
-        write_if_changed(
-            str(docs / "platforms" / f"{name}.md"),
-            generate_platform_page(name, cov, registry, {
-                emu: data for emu, data in emulator_files.items() if emu in runs
-            }),
-        )
-        PAGE_DATA[f"platforms/{name}.md"] = platform_structured_data(name, cov)
-
     print("Generating system pages...")
 
     prime_system_icons({
@@ -6222,16 +6531,55 @@ def _generate(args: argparse.Namespace) -> None:
     console_installs = system_install_choices(coverages)
     (docs / "files").mkdir(parents=True, exist_ok=True)
     write_if_changed(str(docs / "files" / "index.md"), generate_files_page())
+    reset_console_pages()
     for mfr, consoles in manufacturers.items():
         slug = mfr.lower().replace(" ", "-")
-        page = generate_system_page(mfr, consoles, consumers, console_installs.get(slug))
-        PAGE_DATA[f"systems/{slug}.md"] = system_structured_data(slug, consoles)
+        pages = system_pages(mfr, consoles, consumers, console_installs.get(slug))
+        for stem, (page, held) in pages.items():
+            PAGE_DATA[f"systems/{stem}.md"] = system_structured_data(stem, held)
+            if stem != slug:
+                PART_PAGES.add(f"systems/{stem}.md")
+            write_if_changed(str(docs / "systems" / f"{stem}.md"), page)
         write_single_item(
             docs / "api" / "v1" / "systems" / f"{slug}.json", "systems",
             system_export_item(slug, mfr, consoles),
             str(db.get("generated_at") or _timestamp()),
         )
-        write_if_changed(str(docs / "systems" / f"{slug}.md"), page)
+    clash = split_system_stems() & {_slug(m) for m in manufacturers}
+    if clash:
+        raise RuntimeError(f"a console page takes a manufacturer's address: {sorted(clash)}")
+
+    print("Generating static API and bulk data exports...")
+    exports = generate_data_exports(
+        docs, db, coverages, profiles, stats, gap_report
+    )
+    write_if_changed(str(docs / "data.md"), generate_data_page(exports) + REFERENCE_NEXT)
+    PAGE_DATA["data.md"] = dataset_structured_data(exports, db)
+
+    export_reports = load_export_reports()
+    _EXPORTED.update(report["platform"] for report in export_reports)
+
+    print("Generating platform pages...")
+    write_if_changed(
+        str(docs / "platforms" / "index.md"),
+        generate_platform_index(coverages, registry),
+    )
+    for name, cov in coverages.items():
+        # The emulators this platform runs, not every one of the system.
+        runs = resolve_platform_cores(cov["config"], profiles)
+        moved: dict[str, str] = {}
+        write_if_changed(
+            str(docs / "platforms" / f"{name}.md"),
+            generate_platform_page(name, cov, registry, {
+                emu: data for emu, data in emulator_files.items() if emu in runs
+            }, moved),
+        )
+        for stem, page in moved.items():
+            if stem in coverages:
+                raise RuntimeError(f"a system page takes the platform {stem}'s address")
+            PART_PAGES.add(f"platforms/{stem}.md")
+            write_if_changed(str(docs / "platforms" / f"{stem}.md"), page)
+        PAGE_DATA[f"platforms/{name}.md"] = platform_structured_data(name, cov)
 
     print("Generating emulator pages...")
     _DATA_MEMBERS.update(data_directory_members())
@@ -6264,12 +6612,18 @@ def _generate(args: argparse.Namespace) -> None:
                 indent=2, sort_keys=True, ensure_ascii=False, default=str,
             ) + "\n",
         )
-        page = generate_emulator_page(
+        pages = emulator_pages(
             name, profile, db, consumers, suppl_names, data_registry,
             data_dirs=page_data_directories(name, profiles),
         )
         PAGE_DATA[f"emulators/{name}.md"] = emulator_structured_data(profile)
-        write_if_changed(str(docs / "emulators" / f"{name}.md"), page)
+        files = profile.get("files") or []
+        for stem, entries in emulator_parts(name, files):
+            PAGE_DATA[f"emulators/{stem}.md"] = emulator_structured_data(
+                {**profile, "files": [files[i] for i in entries]})
+            PART_PAGES.add(f"emulators/{stem}.md")
+        for stem, page in pages.items():
+            write_if_changed(str(docs / "emulators" / f"{stem}.md"), page)
 
     print("Generating cross-reference page...")
     write_if_changed(
@@ -6323,6 +6677,11 @@ def _generate(args: argparse.Namespace) -> None:
     print("Generating contributing page...")
     write_if_changed(str(docs / "contributing.md"), generate_contributing())
 
+    # Swept before the decoration, which rewrites every page it finds: a
+    # page no generator produced any more (a console moved back onto its
+    # manufacturer's page) was decorated, recorded, and kept for ever.
+    stale = _sweep_generated(docs, GENERATED_DIRS)
+
     print("Adding page metadata and structured data...")
     decorate_markdown_pages(docs)
 
@@ -6421,19 +6780,17 @@ markdown_extensions:
 - tables
 - toc:
     permalink: true
-    # File cards are level 4 headings: the search lands on them, and a
-    # page of a hundred cards keeps a readable table of contents.
+    # Level 3 is the wiki's subsections. On pages of file cards, also level
+    # 3, the hook keeps the table of contents to level 2.
     toc_depth: 3
 - pymdownx.details
 - pymdownx.highlight:
     anchor_linenums: true
 - pymdownx.inlinehilite
 - pymdownx.keys
-- pymdownx.superfences:
-    custom_fences:
-    - name: mermaid
-      class: mermaid
-      format: !!python/name:pymdownx.superfences.fence_code_format
+# No mermaid fence: Material drew it in the browser with a script from a
+# CDN. Diagrams are SVG files written by scripts/sitediagrams.py.
+- pymdownx.superfences
 - pymdownx.tabbed:
     alternate_style: true
 plugins:
@@ -6449,26 +6806,11 @@ validation:
     config_path, config_text = _mkdocs_config(Path(docs), mkdocs_static + nav_yaml)
     write_if_changed(str(config_path), config_text)
 
-    total_pages = (
-        1  # home
-        + 1
-        + len(coverages)  # platform index + detail
-        + 1
-        + len(manufacturers)  # system index + detail
-        + 1  # cross-reference
-        + 1
-        + sum(
-            1 for profile in profiles.values()
-            if profile.get("type") not in ("alias", "test")
-        )  # emulator detail pages (aliases/tests remain metadata-only)
-        + 1  # gap analysis
-        + 1  # which-pack
-        + len(list(Path("wiki").glob("*.md")))  # wiki pages copied verbatim
-        + 1  # generated wiki/data-model
-        + 1  # contributing
-        + 1  # data and API
+    # Counted on disk: the split pages made a sum of page kinds wrong.
+    total_pages = sum(
+        1 for path in docs.rglob("*.md")
+        if not any(part in LOCAL_NOTE_DIRS for part in path.relative_to(docs).parts[:-1])
     )
-    stale = _sweep_generated(docs, GENERATED_DIRS)
     print(f"\nGenerated {total_pages} pages in {args.docs_dir}/")
     if stale:
         print(f"Removed {stale} page(s) nothing produced this run")
@@ -6500,6 +6842,13 @@ from siterender import (  # noqa: E402,F401
     _system_page_map,
     _system_link,
     set_system_page_map,
+    card_page,
+    console_href,
+    record_console_pages,
+    record_emulator_cards,
+    emulator_card_page,
+    reset_console_pages,
+    split_system_stems,
     _platform_link,
     _emulator_link,
     _fmt_size,