[Feature] Persistence contract polish (close, integrity hook, InMemory snapshot keepN, prune policy, ByTag TagFilter)

[pathosDev]

Small contract-consistency fixes: add close?() to the DurableStateStore interface (three impls already have it); add an integrity() hook on DurableStateActor (today PersistenceOptions.integrity is unreachable per-actor); give InMemorySnapshotStore upsert-on-same-seq + optional keepN; standardize the prune-failure policy (best-effort everywhere, like object storage); let ByTagSettings.tag accept a TagFilter. Part of Phase 5.