Stamp the site-wide build time post-render

Hakyll caches per-page outputs, so pages whose dependencies have not
changed are not recompiled and the rendered $build-time$ in their
footer goes stale relative to a fresh build. The right granularity for
"last built at" is site-wide rather than per-page; wrapping the footer
timestamp in <span data-build-time> and rewriting it after Hakyll lets
every page reflect the current build without paying recompilation cost.

* tools/stamp-build-time.py walks _site/**/*.html after Hakyll runs and
  rewrites each element wrapped in [data-build-time] to the same format
  Contexts.hs:buildTimeField emits, so a fresh render and the post-pass
  agree.
* templates/partials/footer.html wraps $build-time$ in
  <span class="footer-build-time" data-build-time>...</span> so the
  sweep has a stable selector.
* Makefile invokes the sweep between embed.py and compress-assets so
  the .gz/.br sidecars include the fresh stamp.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Makefile

@@ -60,6 +60,12 @@ build:
 	else \
 	  echo "Embedding skipped: run 'uv sync' to enable similar-links (build continues)"; \
 	fi
+	# Site-wide footer timestamp: rewrite every <span data-build-time>
+	# in _site/**/*.html so cached (un-recompiled) pages don't show a
+	# stale per-page build time. See tools/stamp-build-time.py for the
+	# full rationale. Must run before compress-assets so the .gz/.br
+	# sidecars include the fresh stamp.
+	@python3 tools/stamp-build-time.py _site
 	@./tools/compress-assets.sh _site
 	> IGNORE.txt
 	@BUILD_END=$$(date +%s); \

templates/partials/footer.html

@@ -7,7 +7,7 @@
         <span class="footer-license"><a href="https://creativecommons.org/licenses/by-nc-sa/4.0/" rel="license">CC&nbsp;BY-NC-SA&nbsp;4.0</a> · <a href="https://git.levineuwirth.org/neuwirth/levineuwirth.org">MIT</a> · <a href="/memento-mori.html" class="footer-mm">MM</a></span>
     </div>
     <div class="footer-right">
-        <a href="/build/" class="footer-build-link" aria-label="Build telemetry">build</a> $build-time$
+        <a href="/build/" class="footer-build-link" aria-label="Build telemetry">build</a> <span class="footer-build-time" data-build-time>$build-time$</span>
         · <a href="$url$.sig" class="footer-sig-link" aria-label="PGP signature for this page" title="Ed25519 signing subkey C9A42A6F AD444FBE 566FD738 531BDC1C C2707066 · public key at /gpg/pubkey.asc">sig</a>
     </div>
 </footer>

tools/stamp-build-time.py

@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+"""Post-build sweep: stamp the site-wide build time into every footer.
+
+Why this exists
+---------------
+build/Contexts.hs binds $build-time$ to getCurrentTime at item-compile
+time, but Hakyll caches outputs. Pages whose dependencies have not
+changed are not recompiled, so the previously-rendered timestamp stays
+on disk and the footer drifts per page. We want one site-wide
+"last built at" stamp, so this script walks _site/**/*.html after
+Hakyll runs and rewrites the contents of every wrapped element.
+
+Format must match build/Contexts.hs:buildTimeField exactly so a fresh
+build (where Hakyll renders the timestamp itself) and the sweep agree.
+"""
+from __future__ import annotations
+
+import os
+import re
+import sys
+from datetime import datetime, timezone
+
+
+def ordinal_suffix(day: int) -> str:
+    if 11 <= day <= 13:
+        return "th"
+    return {1: "st", 2: "nd", 3: "rd"}.get(day % 10, "th")
+
+
+def format_now() -> str:
+    now = datetime.now(timezone.utc)
+    return (
+        f"{now.strftime('%A, %B')} "
+        f"{now.day}{ordinal_suffix(now.day)}, "
+        f"{now.strftime('%Y %H:%M:%S')}"
+    )
+
+
+PATTERN = re.compile(
+    rb'(<span class="footer-build-time" data-build-time>)[^<]*(</span>)'
+)
+
+
+def stamp_file(path: str, replacement_bytes: bytes) -> bool:
+    with open(path, "rb") as f:
+        data = f.read()
+    new_data, count = PATTERN.subn(
+        lambda m: m.group(1) + replacement_bytes + m.group(2),
+        data,
+    )
+    if count and new_data != data:
+        with open(path, "wb") as f:
+            f.write(new_data)
+        return True
+    return False
+
+
+def main(root: str) -> int:
+    if not os.path.isdir(root):
+        print(f"stamp-build-time: {root} not found", file=sys.stderr)
+        return 1
+    timestamp = format_now().encode("utf-8")
+    rewritten = 0
+    scanned = 0
+    for dirpath, _, files in os.walk(root):
+        for name in files:
+            if not name.endswith(".html"):
+                continue
+            scanned += 1
+            if stamp_file(os.path.join(dirpath, name), timestamp):
+                rewritten += 1
+    print(f"stamp-build-time: rewrote {rewritten}/{scanned} HTML files")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv[1] if len(sys.argv) > 1 else "_site"))