11e01b9026
Replace per-message async fire-and-forget with direct-buffer write loop mirroring NatsClient pattern: SpinLock-guarded buffer append, double- buffer swap, single WriteAsync per batch. - MqttConnection: add _directBuf/_writeBuf + RunMqttWriteLoopAsync - MqttConnection: add EnqueuePublishNoFlush (zero-alloc PUBLISH format) - MqttPacketWriter: add WritePublishTo(Span<byte>) + MeasurePublish - MqttTopicMapper: add NatsToMqttBytes with bounded ConcurrentDictionary - MqttNatsClientAdapter: synchronous SendMessageNoFlush + SignalFlush - Skip FlushAsync on plain TCP sockets (TCP auto-flushes)