Adds CultStoredDocument.StoredVersion, CacheBackingStore.VersionOf and the
SingleFileBackingStore.SerializeRecord hook, and swaps every identity comparison
in the same commit: SameStoredIdentity stays the one comparison, over versions.
Deletes the Identity helper, the (schemaId, storedAt) Expected tuple, the
in-memory fake-record projection and the directory's storedAt arrival.
Breaking public API for 2.0.0: CultCommitRequest.Expected, ConditionsHold and
the abstract SingleFileBackingStore.SerializeRecord.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>