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/ConsumerManager.cs

@@ -24,6 +24,12 @@ public sealed class ConsumerManager : IDisposable
     /// </summary>
     public event EventHandler<(string Stream, string Name)>? OnAutoResumed;
 
+    /// <summary>
+    /// Optional reference to the stream manager, used to resolve DeliverPolicy.New
+    /// start sequences at consumer creation time.
+    /// </summary>
+    public StreamManager? StreamManager { get; set; }
+
     public ConsumerManager(JetStreamMetaGroup? metaGroup = null)
     {
         _metaGroup = metaGroup;
@@ -47,6 +53,20 @@ public sealed class ConsumerManager : IDisposable
         if (!JetStreamConfigValidator.IsMetadataWithinLimit(config.Metadata))
             return JetStreamApiResponse.ErrorResponse(400, "consumer metadata exceeds maximum size");
 
+        // Go: DeliverPolicy.New — snapshot the stream's current last sequence at creation
+        // time so the consumer only sees messages published after this point.
+        // Reference: server/consumer.go — setStartingSequenceForDeliverNew.
+        // We set OptStartSeq but preserve DeliverPolicy.New in the stored config;
+        // the fetch engine uses OptStartSeq when set regardless of policy.
+        if (config.DeliverPolicy == DeliverPolicy.New && StreamManager != null)
+        {
+            if (StreamManager.TryGet(stream, out var streamHandle))
+            {
+                var streamState = streamHandle.Store.GetStateAsync(default).GetAwaiter().GetResult();
+                config.OptStartSeq = streamState.LastSeq + 1;
+            }
+        }
+
         if (config.FilterSubjects.Count == 0 && !string.IsNullOrWhiteSpace(config.FilterSubject))
             config.FilterSubjects.Add(config.FilterSubject);
 
@@ -302,6 +322,13 @@ public sealed class ConsumerManager : IDisposable
         return handle.AckProcessor.PendingCount;
     }
 
+    /// <summary>
+    /// Returns true if there are any consumers registered for the given stream.
+    /// Used to short-circuit the LoadAsync call on the publish hot path.
+    /// </summary>
+    public bool HasConsumersForStream(string stream)
+        => _consumers.Keys.Any(k => string.Equals(k.Stream, stream, StringComparison.Ordinal));
+
     public void OnPublished(string stream, StoredMessage message)
     {
         foreach (var handle in _consumers.Values.Where(c => c.Stream == stream && c.Config.Push))