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/Models/StreamConfig.cs

@@ -114,4 +114,16 @@ public sealed class StreamSourceConfig
     // Defaults to 0 (disabled). When > 0, duplicate messages with the same Nats-Msg-Id
     // within this window are silently dropped.
     public int DuplicateWindowMs { get; set; }
+
+    // Go: StreamSource.SubjectTransforms — per-source subject transforms.
+    // Reference: golang/nats-server/server/stream.go — SubjectTransforms field.
+    public List<SubjectTransformConfig> SubjectTransforms { get; set; } = [];
+}
+
+// Go: SubjectTransformConfig — source/destination subject transform pair.
+// Reference: golang/nats-server/server/stream.go — SubjectTransformConfig struct.
+public sealed class SubjectTransformConfig
+{
+    public string Source { get; set; } = string.Empty;
+    public string Destination { get; set; } = string.Empty;
 }