feat: add atomic file writer with SemaphoreSlim for crash-safe state writes (Gap 1.6)

- 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

src/NATS.Server/JetStream/Storage/IStreamStore.cs

@@ -56,7 +56,7 @@ public interface IStreamStore
         => throw new NotSupportedException("Block-engine SkipMsgs not yet implemented.");
 
     // Go: StreamStore.FlushAllPending — flush any buffered writes to backing storage
-    void FlushAllPending()
+    Task FlushAllPending()
         => throw new NotSupportedException("Block-engine FlushAllPending not yet implemented.");
 
     // Go: StreamStore.LoadMsg — load message by exact sequence; sm is an optional reusable buffer