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/NatsServer.cs

@@ -1694,7 +1694,14 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                     var numPending = batch - delivered;
                     var ackReply = BuildAckReply(ackPrefix, message.Sequence, deliverySeq, tsNanos, numPending);
 
-                    DeliverMessage(inboxSub, message.Subject, ackReply, minHeaders, message.Payload);
+                    // Bypass DeliverMessage — we already know the target client is sender.
+                    // Skip permission check and auto-unsub overhead for JS delivery inbox.
+                    // Go reference: consumer.go — batch delivery with deferred flush.
+                    sender.SendMessageNoFlush(message.Subject, inboxSub.Sid, ackReply, minHeaders, message.Payload);
+
+                    // Batch flush every 64 messages to amortize write-loop wakeup cost.
+                    if ((delivered & 63) == 0)
+                        sender.SignalFlush();
 
                     if (consumer.Config.AckPolicy is JetStream.Models.AckPolicy.Explicit or JetStream.Models.AckPolicy.All)
                     {
@@ -1708,6 +1715,11 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 }
                 else
                 {
+                    // Flush any buffered messages before blocking on the signal.
+                    // Without this, messages queued via SendMessageNoFlush would sit
+                    // in the buffer until the next batch boundary or loop exit.
+                    sender.SignalFlush();
+
                     // No message available — send idle heartbeat if needed
                     if (DateTime.UtcNow - lastDeliveryTime >= hbInterval)
                     {
@@ -1719,8 +1731,17 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                         lastDeliveryTime = DateTime.UtcNow;
                     }
 
-                    // Poll briefly before retrying
-                    await Task.Delay(5, ct).ConfigureAwait(false);
+                    // Wait for publisher to signal a new message, with a heartbeat-interval
+                    // timeout so the heartbeat check is re-evaluated periodically.
+                    // Go reference: consumer.go — channel signaling from publisher.
+                    try
+                    {
+                        await streamHandle.WaitForPublishAsync(ct).WaitAsync(hbInterval, ct).ConfigureAwait(false);
+                    }
+                    catch (TimeoutException)
+                    {
+                        // Heartbeat interval elapsed — loop back to re-check
+                    }
                 }
             }
         }
@@ -1729,13 +1750,16 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             // ExpiresMs timeout — expected
         }
 
+        // Final flush for any remaining batched messages
+        sender.SignalFlush();
+
         consumer.NextSequence = sequence;
 
-        // Send terminal status
+        // Send terminal status directly to sender (last message, includes flush)
         ReadOnlyMemory<byte> statusHeader = delivered == 0
             ? System.Text.Encoding.UTF8.GetBytes("NATS/1.0 404 No Messages\r\n\r\n")
             : System.Text.Encoding.UTF8.GetBytes("NATS/1.0 408 Request Timeout\r\n\r\n");
-        DeliverMessage(inboxSub, replyTo, null, statusHeader, default);
+        sender.SendMessage(replyTo, inboxSub.Sid, null, statusHeader, default);
     }
 
     private void DeliverFetchedMessages(Subscription inboxSub, string streamName, string consumerName,