perf: batch flush + signal-based wakeup for JS pull consumers

Replace per-message DeliverMessage/flush in DeliverPullFetchMessagesAsync
with SendMessageNoFlush + batch flush every 64 messages. Add signal-based
wakeup (StreamHandle.NotifyPublish/WaitForPublishAsync) to replace 5ms
Task.Delay polling in both DeliverPullFetchMessagesAsync and
PullConsumerEngine.WaitForMessageAsync. Publishers signal waiting
consumers immediately after store append.

src/NATS.Server/JetStream/StreamManager.cs

@@ -457,6 +457,9 @@ public sealed class StreamManager : IDisposable
         var seq = stream.Store.AppendAsync(storeSubject, payload, default).GetAwaiter().GetResult();
         EnforceRuntimePolicies(stream, DateTime.UtcNow);
 
+        // Wake up any pull consumers waiting at the stream tail.
+        stream.NotifyPublish();
+
         // Only load the stored message when replication is configured (mirror/source).
         // Avoids unnecessary disk I/O on the hot publish path.
         if (_mirrorsByOrigin.ContainsKey(stream.Config.Name) || _sourcesByOrigin.ContainsKey(stream.Config.Name))
@@ -507,6 +510,9 @@ public sealed class StreamManager : IDisposable
         var seq = stream.Store.AppendAsync(storeSubject, newPayload, default).GetAwaiter().GetResult();
         EnforceRuntimePolicies(stream, DateTime.UtcNow);
 
+        // Wake up any pull consumers waiting at the stream tail.
+        stream.NotifyPublish();
+
         if (_mirrorsByOrigin.ContainsKey(stream.Config.Name) || _sourcesByOrigin.ContainsKey(stream.Config.Name))
         {
             var stored = stream.Store.LoadAsync(seq, default).GetAwaiter().GetResult();
@@ -961,4 +967,25 @@ public sealed class StreamManager : IDisposable
     }
 }
 
-public sealed record StreamHandle(StreamConfig Config, IStreamStore Store);
+public sealed record StreamHandle(StreamConfig Config, IStreamStore Store)
+{
+    // Signal-based wakeup for pull consumers waiting at the stream tail.
+    // Go reference: consumer.go — channel signaling from publisher to waiting consumer.
+    private volatile TaskCompletionSource _publishSignal = new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+    /// <summary>
+    /// Notifies waiting consumers that a new message has been published.
+    /// </summary>
+    public void NotifyPublish()
+    {
+        var old = Interlocked.Exchange(ref _publishSignal,
+            new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously));
+        old.TrySetResult();
+    }
+
+    /// <summary>
+    /// Waits until a new message is published to this stream.
+    /// </summary>
+    public Task WaitForPublishAsync(CancellationToken ct)
+        => _publishSignal.Task.WaitAsync(ct);
+}