Merge branch 'opt/js-async-file-publish'

JetStream async file publish optimization (~10% improvement):
- Cached state properties eliminate GetStateAsync on publish path
- Single stream lookup eliminates double FindBySubject
- Removed _messageIndexes dictionary from write path
- Hand-rolled UTF-8 pub-ack formatter for success path
- Exponential flush backoff matching Go server
- Lazy StoredMessage materialization (MessageMeta struct)

# Conflicts:
#	benchmarks_comparison.md

src/NATS.Server/NatsServer.cs

@@ -1399,8 +1399,19 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             // Go reference: server/jetstream.go — jsPubAckResponse sent to reply.
             if (replyTo != null)
             {
-                var ackData = JsonSerializer.SerializeToUtf8Bytes(pubAck, s_jetStreamJsonOptions);
-                ProcessMessage(replyTo, null, default, ackData, sender);
+                if (JetStream.Publish.JetStreamPubAckFormatter.IsSimpleSuccess(pubAck))
+                {
+                    // Fast path: hand-rolled UTF-8 formatter avoids JsonSerializer overhead.
+                    Span<byte> ackBuf = stackalloc byte[256];
+                    var ackLen = JetStream.Publish.JetStreamPubAckFormatter.FormatSuccess(ackBuf, pubAck.Stream, pubAck.Seq);
+                    ProcessMessage(replyTo, null, default, ackBuf[..ackLen].ToArray(), sender);
+                }
+                else
+                {
+                    var ackData = JsonSerializer.SerializeToUtf8Bytes(pubAck, s_jetStreamJsonOptions);
+                    ProcessMessage(replyTo, null, default, ackData, sender);
+                }
+
                 return;
             }
         }