Lookup index auto-invalidates on KB updates + manual rebuild flag

Diego asked: does the persisted ``archcenter-refs-index.json`` auto-
update when the KB is updated? Partially — mtime on the cache root
catches add/remove of subdirs (POSIX), but NOT changes to files
INSIDE existing subdirs (description.md added, .drawio replaced).

Two fixes:

1. Both KB-update tools now ``touch`` ``kb/diagram/assets/archcenter-refs``
   when they finish a run that produced changes:
   - ``archcenter_description_fetcher.py`` after fetched > 0
   - ``archcenter_zip_downloader.py`` after downloaded / extract /
     overwrite events
   The touch bumps the parent mtime, which the next lookup detects
   and uses to rebuild its persisted index. Closes the silent-drift
   case where description.md fetches landed but the index didn't
   notice.

2. ``archcenter_pattern_lookup.py --rebuild-index`` flag — escape
   hatch for manual KB edits (someone replaces a cached .drawio by
   hand) where neither tool ran. Removes the stale index file before
   the lookup, forcing a cold rebuild.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/archcenter_zip_downloader.py

@@ -211,6 +211,16 @@ def main() -> None:
         print(f"  {status:30s}  {n}", file=sys.stderr)
     print(f"\nReport: {args.report.relative_to(PROJECT_ROOT)}", file=sys.stderr)
 
+    # Bump cache-dir mtime so the pattern-lookup index invalidates.
+    # New downloads create new subdirs (POSIX bumps the parent mtime),
+    # but re-downloads or unzip-overwrites of existing slugs do not —
+    # so we touch explicitly.
+    if any(counts.get(k) for k in ("downloaded", "zip_error", "no_drawio_after_extract")):
+        try:
+            args.cache_dir.touch(exist_ok=True)
+        except OSError:
+            pass
+
 
 if __name__ == "__main__":
     main()