Add CacheBackingStore.ReadDurable (single-file and directory stores), factor the
directory pull's lease/retry load into LoadManifestPages, and make a store-backed
Update take apply's argument and the Expected version from that read, with the load's
deterministic element ids. The per-loss pull moves to the LostRace branch; a
store-less cache keeps the held copy and its codec refusal.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>