FdbSnapshot names copies cdclient-<hash>.fdb / CDServer-<hash>.sqlite, writes
them under temporary names and renames them in, keeps a pointer file naming
the current pair, removes old copies, and decides when a changed file has
settled enough to hash. Nothing in it logs, so it can run on a worker.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>