- Add AtomicFileWriter static helper: writes to {path}.{random}.tmp, flushes,
then File.Move(overwrite:true) — concurrent-safe via unique temp path per call
- Add _stateWriteLock (SemaphoreSlim 1,1) to FileStore; dispose in both Dispose
and DisposeAsync paths
- Promote WriteStreamState to async WriteStreamStateAsync using AtomicFileWriter
under the write lock; FlushAllPending now returns Task
- Update IStreamStore.FlushAllPending signature to Task; fix MemStore no-op impl
- Fix FileStoreCrashRecoveryTests to await FlushAllPending (3 sync→async tests)
- Add 9 AtomicFileWriterTests covering create, no-tmp-remains, overwrite,
concurrent safety, memory overload, empty data, and large payload
Add FlushAllPending() to FileStore, fulfilling the IStreamStore interface
contract. The method flushes the active MsgBlock to disk and atomically
writes a stream.state checkpoint using write-to-temp + rename, matching
Go's flushPendingWritesUnlocked / writeFullState pattern.
Add FileStoreCrashRecoveryTests with 5 tests covering:
- FlushAllPending flushes block data to .blk file
- FlushAllPending writes a valid atomic stream.state JSON checkpoint
- FlushAllPending is idempotent (second call overwrites with latest state)
- Recovery prunes messages backdated past the MaxAgeMs cutoff
- Recovery handles a tail-truncated block without throwing
Reference: golang/nats-server/server/filestore.go:5783-5842
