fix: ignore the rendered timestamp when comparing

Decorated site pages carry the generation stamp twice: once as the
markdown footer and once as a rendered element. write_if_changed knew
only the first, so every page was rewritten on every run for the clock
alone.

The comparison is what makes the deploy-site freshness guard a real
staleness check rather than a guaranteed failure, and it had no tests.

scripts/common.py

@@ -1368,6 +1368,9 @@ _TIMESTAMP_PATTERNS = [
     re.compile(r'"imported_at":\s*"[^"]*"'),  # provenance snapshots
     re.compile(r"\*Auto-generated on [^*]*\*"),  # README.md
     re.compile(r"\*Generated on [^*]*\*"),  # docs site pages
+    # The decorated pages carry the same stamp again as a rendered element,
+    # and missing it rewrote every page on every run for the clock alone.
+    re.compile(r'<div class="rb-timestamp">[^<]*</div>'),
 ]