fix: MqttClient 回退信号方案，添加 publish debug 日志追踪阻塞点

- 移除 QObject 基类和 QueuedConnection 信号方案
- publish() 直接调用 self._client.publish() 并打印前后日志
- 添加 print(flush=True) 确保 Windows 终端实时输出

目的是确认阻塞发生在:
  [MqttClient] publish topic=... qos=1  ← 打印后消失？
  [MqttClient] publish done mid=...      ← 这行能否出现？

DBNMQTTool/main.py

@@ -39,6 +39,8 @@ from dbn_mqtt_tool.protocol import (
 )
 
 class MainWindow(QMainWindow):
+    _mqtt_status = Signal(bool, str)
+    _mqtt_msg = Signal(str, str, object)
     _devices_changed = Signal()
 
     def __init__(self):
@@ -50,10 +52,12 @@ class MainWindow(QMainWindow):
         self._mqtt = MqttClient()
         self._devmgr = DeviceManager()
 
-        self._mqtt._status_notify.connect(self._on_status)
-        self._mqtt._msg_notify.connect(self._on_message)
+        self._mqtt.on_status_change(lambda c, m: self._mqtt_status.emit(c, m))
+        self._mqtt.on_message(lambda t, s, p: self._mqtt_msg.emit(t, s, p))
         self._devmgr.on_change(lambda: self._devices_changed.emit())
 
+        self._mqtt_status.connect(self._on_status)
+        self._mqtt_msg.connect(self._on_message)
         self._devices_changed.connect(self._refresh_devices)
 
         self._build_ui()