refactor: emit site blocks through sitemarkup

Callouts, collapsible blocks, tabs, buttons, card grids and search
exclusion go through one module that writes the engine's syntax. The
MkDocs output is unchanged apart from whitespace-only lines.

scripts/generate_readme.py

@@ -230,14 +230,16 @@ def release_totals(platform_name: str, record: dict) -> tuple[int | None, int |
 
 
 def download_links(
-    platform_name: str, record: dict, repo: str, label: str, attrs: str = ""
+    platform_name: str, record: dict, repo: str, label: str, attrs: str = "",
+    button: Callable[[str, str], str] | None = None,
 ) -> str:
     """Markdown links to the files of a platform's pack in the recorded release.
 
     A pack published whole gets one link labelled *label*. A pack in parts
     gets one link per part, since every part is needed. *repo* is the
     repository URL, absolute or relative, and *attrs* a MkDocs attribute list
-    put after each link. Without a recorded release the link is the latest
+    put after each link, or *button* the function that draws each link as a
+    button on the site. Without a recorded release the link is the latest
     release page. A release recorded without this platform's pack says so and
     names the installer, which reads the file lists on main: a button to the
     release served six platforms a page with no pack for them.
@@ -245,18 +247,22 @@ def download_links(
     pack = release_record.pack_for(platform_name, record)
     tag = record.get("tag")
     assets = pack.get("assets") if pack else None
+    def link(text: str, url: str) -> str:
+        return button(text, url) if button else f"[{text}]({url}){attrs}"
+
     if not tag:
-        return f"[{label}]({repo}/releases/latest){attrs}"
+        return link(label, f"{repo}/releases/latest")
     if not assets:
         return f"Not in {tag}: [use the installer]({SITE_URL}wiki/installer/)"
     urls = [f"{repo}/releases/download/{tag}/{asset}" for asset in assets]
     if len(urls) == 1:
-        return f"[{label}]({urls[0]}){attrs}"
+        return link(label, urls[0])
+    styled = bool(attrs or button)
     parts = [
-        f"[{'P' if attrs else 'p'}art {number} of {len(urls)}]({url}){attrs}"
+        link(f"{'P' if styled else 'p'}art {number} of {len(urls)}", url)
         for number, url in enumerate(urls, 1)
     ]
-    if attrs:
+    if styled:
         return " ".join(parts)
     return f"{label}: " + ", ".join(parts)