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/siterender.py

@@ -139,13 +139,75 @@ def _page_by_norm(sys_id: str) -> tuple[str, str] | None:
     return _norm_page_map.get(_norm_system_id(sys_id))
 
 
+# A manufacturer page too large for one page is a summary, each console on
+# pages of its own: (slug, console) -> its page stems under systems/, and
+# (slug, console, sha1) -> the stem holding that card. A console of a page
+# kept whole has no entry, its section is on the manufacturer's page.
+_console_pages: dict[tuple[str, str], list[str]] = {}
+_card_pages: dict[tuple[str, str, str], str] = {}
+
+
+def reset_console_pages() -> None:
+    _console_pages.clear()
+    _card_pages.clear()
+
+
+def record_console_pages(
+    slug: str, console: str, stems: list[str], sha1s: list[list[str]]
+) -> None:
+    """Where a split manufacturer page put one console: its page stems, in
+    order, and the sha1 of each card on each of them."""
+    _console_pages[(slug, console)] = list(stems)
+    for stem, part in zip(stems, sha1s):
+        for sha1 in part:
+            _card_pages.setdefault((slug, console, sha1), stem)
+
+
+def split_system_stems() -> set[str]:
+    """Every page stem of a console moved to a page of its own."""
+    return {stem for stems in _console_pages.values() for stem in stems}
+
+
+def console_href(slug: str, console: str, prefix: str = "") -> str:
+    """Where a console's files are: its own page, or its section of the
+    manufacturer's page (also where a console cut into parts lists them)."""
+    stems = _console_pages.get((slug, console))
+    if stems and len(stems) == 1:
+        return f"{prefix}systems/{urllib.parse.quote(stems[0])}.md"
+    return f"{prefix}systems/{urllib.parse.quote(slug)}.md#{_slugify_anchor(console)}"
+
+
+def card_page(path: str, sha1: str) -> str:
+    """The systems/ page stem holding the card of the copy at ``path``
+    (bios/<Manufacturer>/<Console>/...)."""
+    parts = path.split("/")
+    slug = parts[1].lower().replace(" ", "-")
+    console = parts[2] if len(parts) > 2 else ""
+    return _card_pages.get((slug, console, sha1), slug)
+
+
+# Emulator cards moved off a page too long for one: (emulator, anchor) ->
+# the page stem under emulators/ that holds the card.
+_emulator_card_pages: dict[tuple[str, str], str] = {}
+
+
+def record_emulator_cards(emulator: str, stem: str, anchors: list[str]) -> None:
+    for anchor in anchors:
+        _emulator_card_pages[(emulator, anchor)] = stem
+
+
+def emulator_card_page(emulator: str, anchor: str) -> str:
+    """The emulators/ page stem holding a card: the emulator's own page, or
+    the page its cards went to."""
+    return _emulator_card_pages.get((emulator, anchor), emulator)
+
+
 def _system_link(sys_id: str, prefix: str = "") -> str:
     """Generate a markdown link to a system page with anchor."""
     page = _system_page_map.get(sys_id) or _page_by_norm(sys_id)
     if page:
         slug, console = page
-        anchor = _slugify_anchor(console)
-        return f"[{sys_id}]({prefix}systems/{slug}.md#{anchor})"
+        return f"[{sys_id}]({console_href(slug, console, prefix)})"
     return sys_id
 
 def _field_text(key: object, value: object) -> str: