fix: MqttClient 重构为 QObject — publish 通过 Qt 信号排队避免 C 层崩溃

问题: paho-mqtt publish() C 扩展在 Windows 上从 GUI 线程调用时静默崩溃。

修复:
- MqttClient 改为 QObject 子类
- publish() 通过 Qt Signal(Qt.QueuedConnection) 排队，_do_publish()
  始终在主线程事件循环中执行 paho 调用
- 状态/消息通知改用 Qt Signal 跨线程传递
- 新增 subscribe()/unsubscribe() 公开 API，消除 main.py 直接
  访问 _client 的脆弱代码
- 移除 MainWindow 中已不再需要的中间信号 _mqtt_status/_mqtt_msg

DBNMQTTool/main.py

@@ -39,8 +39,6 @@ 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):
@@ -52,12 +50,10 @@ class MainWindow(QMainWindow):
         self._mqtt = MqttClient()
         self._devmgr = DeviceManager()
 
-        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._mqtt._status_notify.connect(self._on_status)
+        self._mqtt._msg_notify.connect(self._on_message)
         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()
@@ -695,7 +691,7 @@ class MainWindow(QMainWindow):
             self._log(f"[自定义] 发布失败: {e}")
 
     def _custom_subscribe(self):
-        if not self._mqtt.connected or not self._mqtt._client:
+        if not self._mqtt.connected:
             QMessageBox.warning(self, "提示", "请先连接 Broker")
             return
         topic = self._custom_sub_topic.text()
@@ -703,7 +699,7 @@ class MainWindow(QMainWindow):
             return
         try:
             qos = int(self._custom_sub_qos.currentText())
-            self._mqtt._client.subscribe(topic, qos=qos)
+            self._mqtt.subscribe(topic, qos=qos)
             self._custom_subs[topic] = qos
             # 更新列表
             found = False
@@ -724,13 +720,11 @@ class MainWindow(QMainWindow):
         items = self._custom_sub_list.selectedItems()
         if not items:
             return
-        if not self._mqtt._client:
-            return
         for item in items:
             topic = item.text(0)
             try:
                 if topic in self._custom_subs:
-                    self._mqtt._client.unsubscribe(topic)
+                    self._mqtt.unsubscribe(topic)
                     del self._custom_subs[topic]
                 idx = self._custom_sub_list.indexOfTopLevelItem(item)
                 self._custom_sub_list.takeTopLevelItem(idx)