Standing rule this file exists to enforce. Heavily-worked files stop writing their own handoff entries. This registry already caught two real drifts — the SAVE_VERSION 10→21 gap, and a rename pass that silently missed two docs that weren't uploaded that day. Resync it every time a session materially changes state.
Vintage check: if a file on disk doesn't match the KB below, it's a stale download — re-pull it.