- 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
