feat(storage): port FileStore Go tests and add sync methods (Go parity)
Add 67 Go-parity tests from filestore_test.go covering:
- SkipMsg/SkipMsgs sequence reservation
- RemoveMsg/EraseMsg soft-delete
- LoadMsg/LoadLastMsg/LoadNextMsg message retrieval
- AllLastSeqs/MultiLastSeqs per-subject last sequences
- SubjectForSeq reverse lookup
- NumPending with filters and last-per-subject mode
- Recovery watermark preservation after purge
- FastState NumDeleted/LastTime correctness
- PurgeEx with empty subject + keep parameter
- Compact _first watermark tracking
- Multi-block operations and state verification
Implements missing IStreamStore sync methods on FileStore:
RemoveMsg, EraseMsg, SkipMsg, SkipMsgs, LoadMsg, LoadLastMsg,
LoadNextMsg, AllLastSeqs, MultiLastSeqs, SubjectForSeq, NumPending.
Adds MsgBlock.WriteSkip() for tombstone sequence reservation.
Adds IDisposable to FileStore for synchronous test disposal.
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