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.

src/NATS.Server/JetStream/Api/Handlers/StreamApiHandlers.cs

@@ -63,6 +63,11 @@ public static class StreamApiHandlers
         if (streamName == null)
             return JetStreamApiResponse.NotFound(subject);
 
+        // Go: stream update rejects unknown stream names — must exist before update.
+        // Reference: server/jetstream_api.go — jsStreamUpdateT checks stream existence.
+        if (!streamManager.TryGet(streamName, out _))
+            return JetStreamApiResponse.ErrorResponse(404, "stream not found");
+
         var config = ParseConfig(payload);
         if (string.IsNullOrWhiteSpace(config.Name))
             config.Name = streamName;