docs site: content-hash the site's own stylesheets (cache-proof URLs)

brand.css and typography.css are referenced at fixed URLs via extra_css, so a
warm browser cache could keep serving the previous stylesheet after a deploy.
Add on_post_build to publishing/hooks.py: at build time copy each extra_css
file to <name>.<sha256-8>.css in the output and rewrite the <link> in the built
HTML. Any content change now yields a new URL. Sources keep their names; hashed
copies exist only in the build output and stale ones are pruned on rebuild.

publishing/hooks.py

@@ -1,16 +1,32 @@
-"""MkDocs hook: make directory-style links work on a static site.
+"""MkDocs hooks for the publishing layer.
 
-The documents sometimes link to a directory the way Gitea renders it, e.g.
-infrastructure.md contains [`incidents/`](incidents/). On a static site with
-directory URLs, that href resolves relative to the *page* (giving
-/infrastructure/incidents/ and a 404). This hook rewrites such links to the
-generated landing page for that directory (build.sh creates
-<dir>/index.md for documented directories that lack one). It never edits the
-documents themselves.
+Two jobs, both publishing-layer only — neither edits the documents or the
+authored stylesheets in the repository.
+
+1. Directory-style links. The documents sometimes link to a directory the way
+   Gitea renders it, e.g. infrastructure.md contains [`incidents/`](incidents/).
+   On a static site with directory URLs, that href resolves relative to the
+   *page* (giving /infrastructure/incidents/ and a 404). This hook rewrites such
+   links to the generated landing page for that directory (build.sh creates
+   <dir>/index.md for documented directories that lack one).
+
+2. Content-hashed stylesheet URLs. The site's own stylesheets (brand.css,
+   typography.css) are listed in mkdocs.yml under extra_css, so every page
+   links them at a FIXED URL (publishing/brand.css). A fixed URL means a browser
+   with a warm cache keeps serving the old stylesheet after a deploy — the URL
+   never changes even when the content does. Material's own assets
+   (assets/stylesheets/main.<hash>.min.css) are content-hashed and immune; ours
+   were not. This hook gives ours the same property: at build time it copies
+   each to <name>.<sha256-8>.css in the output and rewrites the links in the
+   built HTML, so any change to a stylesheet's content necessarily yields a new
+   URL. The authored sources keep their names and locations.
 """
 
+import hashlib
+import os
 import posixpath
 import re
+import shutil
 
 from mkdocs.utils import get_relative_url
 
@@ -36,3 +52,93 @@ def on_page_content(html, page, config, files):
         return f'{match.group(1)}"{get_relative_url(index_urls[target], page.file.url)}"'
 
     return _HREF.sub(fix, html)
+
+
+# --- Content-hashed stylesheet URLs ----------------------------------------
+
+# Material's own asset names use 8 hex characters (main.57a0d79d.min.css); match
+# that so the site's naming is uniform and the hash is short but collision-safe
+# for a handful of stylesheets.
+_HASH_LEN = 8
+# A hashed sibling of brand.css is brand.<8 hex>.css.
+_HASHED_SIBLING = re.compile(r"^(?P<stem>.+)\.(?P<hash>[0-9a-f]{%d})(?P<ext>\.[^.]+)$" % _HASH_LEN)
+
+
+def _hashed_links(config, files=None):
+    """Build each extra_css file to a content-hashed name in the site output.
+
+    Returns {original-relative-path: hashed-relative-path}. The hashed copy
+    lands in the *same directory* as the original, so relative references
+    inside the CSS (e.g. brand.css's @font-face url("fonts/figtree-400.woff2"))
+    keep resolving. Stale hashed siblings from an earlier build are removed, so
+    the output does not accumulate files across builds.
+    """
+    site_dir = config["site_dir"]
+    replacements = {}
+
+    for css in config.get("extra_css") or []:
+        rel = css.lstrip("/").replace("\\", "/")
+        original = os.path.join(site_dir, *rel.split("/"))
+        if not os.path.isfile(original):
+            continue
+
+        with open(original, "rb") as fh:
+            digest = hashlib.sha256(fh.read()).hexdigest()[:_HASH_LEN]
+
+        stem, ext = posixpath.splitext(rel)
+        hashed_rel = f"{stem}.{digest}{ext}"
+        hashed = os.path.join(site_dir, *hashed_rel.split("/"))
+
+        # Copy to the hashed name (a no-op if it is already correct).
+        if os.path.abspath(hashed) != os.path.abspath(original):
+            shutil.copyfile(original, hashed)
+
+        # Drop stale hashed siblings of the original (same stem/ext, any hash).
+        directory = os.path.dirname(original)
+        base_stem, base_ext = posixpath.splitext(os.path.basename(rel))
+        keep = os.path.basename(hashed)
+        for name in os.listdir(directory):
+            m = _HASHED_SIBLING.match(name)
+            if not m or name == keep:
+                continue
+            if m.group("stem") == base_stem and m.group("ext") == base_ext:
+                os.remove(os.path.join(directory, name))
+
+        replacements[rel] = hashed_rel
+
+    return replacements
+
+
+def _rewrite_stylesheet_links(site_dir, replacements):
+    """Point every <link href="…brand.css"> at the hashed file, in place.
+
+    The href is relative to each page (root pages use "publishing/brand.css",
+    nested pages "../publishing/brand.css"), so match any prefix up to the
+    known relative path and swap only the path tail, preserving the page's
+    own relative prefix.
+    """
+    patterns = [
+        (re.compile(r'(href="[^"]*)' + re.escape(orig) + r'(")'), hashed)
+        for orig, hashed in replacements.items()
+    ]
+
+    for root, _dirs, filenames in os.walk(site_dir):
+        for name in filenames:
+            if not name.endswith(".html"):
+                continue
+            path = os.path.join(root, name)
+            with open(path, "r", encoding="utf-8") as fh:
+                html = fh.read()
+            new = html
+            for pattern, hashed in patterns:
+                new = pattern.sub(lambda m, h=hashed: m.group(1) + h + m.group(2), new)
+            if new != html:
+                with open(path, "w", encoding="utf-8") as fh:
+                    fh.write(new)
+
+
+def on_post_build(config):
+    replacements = _hashed_links(config)
+    if replacements:
+        _rewrite_stylesheet_links(config["site_dir"], replacements)
+