perf: Phase 1 JetStream async file publish optimizations

- Add cached state properties (LastSeq, MessageCount, TotalBytes, FirstSeq)
  to IStreamStore/FileStore/MemStore — eliminates GetStateAsync on publish path
- Add Capture(StreamHandle, ...) overload to StreamManager — eliminates
  double FindBySubject lookup (once in JetStreamPublisher, once in Capture)
- Remove _messageIndexes dictionary from FileStore write path — all lookups
  now use _messages directly, saving ~48B allocation per message
- Add JetStreamPubAckFormatter for hand-rolled UTF-8 success ack formatting —
  avoids JsonSerializer overhead on the hot publish path
- Switch flush loop to exponential backoff (1→2→4→8ms) matching Go server

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;
             }
         }