perf: add FileStore buffered writes, O(1) state tracking, and eliminate redundant per-publish work

Implement Go-parity background flush loop (coalesce 16KB/8ms) in MsgBlock/FileStore,
replace O(n) GetStateAsync with incremental counters, skip PruneExpired/LoadAsync/
PrunePerSubject when not needed, and bypass RAFT for single-replica streams. Fix counter
tracking bugs in RemoveMsg/EraseMsg/TTL expiry and ObjectDisposedException races in
flush loop disposal. FileStore optimizations verified with 3112/3112 JetStream tests
passing; async publish benchmark remains at ~174 msg/s due to E2E protocol path bottleneck.

tests/NATS.Server.TestUtilities/JetStreamClusterFixture.cs

@@ -122,11 +122,13 @@ public sealed class JetStreamClusterFixture : IAsyncDisposable
     /// </summary>
     public JetStreamApiResponse UpdateStream(string name, string[] subjects, int replicas, int maxMsgs = 0)
     {
+        // Go: stream update rejects unknown stream names — must exist before update.
+        if (!_streamManager.TryGet(name, out var existing))
+            return JetStreamApiResponse.ErrorResponse(404, "stream not found");
+
         // Preserve the existing stream's retention policy so ValidateConfigUpdate
         // does not reject the update for changing an immutable field.
-        var retention = RetentionPolicy.Limits;
-        if (_streamManager.TryGet(name, out var existing))
-            retention = existing.Config.Retention;
+        var retention = existing.Config.Retention;
 
         return _streamManager.CreateOrUpdate(new StreamConfig
         {