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/dbn_mqtt_tool/mqtt_client.py

@@ -3,12 +3,12 @@ MQTT 客户端封装
 """
 import json
 import time
+import sys
 import threading
 from typing import Callable, Optional
 from dataclasses import dataclass
 
 import paho.mqtt.client as mqtt
-from PySide6.QtCore import QObject, Signal, Slot, Qt
 
 from .protocol import parse_topic_dev_serial
 
@@ -23,64 +23,67 @@ class BrokerConfig:
     keepalive: int = 60
 
 
-class MqttClient(QObject):
-    """DLD960 IoT MQTT 客户端 — QObject 基类确保信号线程安全"""
-
-    # 供外部监听
-    _status_notify = Signal(bool, str)   # connected, message
-    _msg_notify = Signal(str, str, object)  # topic, dev_serial, payload_dict
-
-    # 内部线程安全的 publish 队列
-    _publish_sig = Signal(str, str, int)  # topic, json_data, qos
+class MqttClient:
+    """DLD960 IoT MQTT 客户端"""
 
     def __init__(self):
-        super().__init__()
         self._client: Optional[mqtt.Client] = None
         self._config = BrokerConfig()
         self._connected = False
         self._lock = threading.Lock()
-
-        # 信号→槽: publish 通过排队连接确保在主线程的 paho context 执行
-        self._publish_sig.connect(self._do_publish, Qt.QueuedConnection)
+        self._message_callbacks: list[Callable[[str, str, dict], None]] = []
+        self._status_callbacks: list[Callable[[bool, str], None]] = []
 
     @property
     def connected(self) -> bool:
         return self._connected
 
-    # ---- 公开回调 (兼容旧接口) ----
-
     def on_status_change(self, callback: Callable[[bool, str], None]):
-        self._status_notify.connect(callback)
+        self._status_callbacks.append(callback)
 
     def on_message(self, callback: Callable[[str, str, dict], None]):
         """回调参数: (topic, dev_serial, payload_dict)"""
-        self._msg_notify.connect(callback)
+        self._message_callbacks.append(callback)
 
-    # ---- MQTT callbacks (paho 网络线程) ----
+    def _notify_status(self, connected: bool, msg: str):
+        for cb in self._status_callbacks:
+            try:
+                cb(connected, msg)
+            except Exception:
+                pass
+
+    def _notify_message(self, topic: str, payload: dict):
+        dev_serial = parse_topic_dev_serial(topic) or ""
+        for cb in self._message_callbacks:
+            try:
+                cb(topic, dev_serial, payload)
+            except Exception:
+                pass
+
+    # ---- MQTT callbacks ----
 
     def _on_connect(self, client, userdata, flags, reason_code, properties=None):
         rc = reason_code if isinstance(reason_code, int) else reason_code.value
         if rc == 0:
             self._connected = True
-            self._status_notify.emit(True, "已连接到 Broker")
+            self._notify_status(True, "已连接到 Broker")
 
             from .protocol import TOPIC_ALL_DEVICE_UP
             client.subscribe(TOPIC_ALL_DEVICE_UP, qos=1)
         else:
             self._connected = False
-            self._status_notify.emit(False, f"连接失败 (rc={rc})")
+            self._notify_status(False, f"连接失败 (rc={rc})")
 
     def _on_disconnect(self, client, userdata, *args):
         self._connected = False
-        self._status_notify.emit(False, "已断开连接")
+        self._notify_status(False, "已断开连接")
 
     def _on_message(self, client, userdata, msg: mqtt.MQTTMessage):
         try:
             payload = json.loads(msg.payload.decode("utf-8"))
         except (json.JSONDecodeError, UnicodeDecodeError):
             payload = {"_raw": msg.payload.hex()}
-        dev_serial = parse_topic_dev_serial(msg.topic) or ""
-        self._msg_notify.emit(msg.topic, dev_serial, payload)
+        self._notify_message(msg.topic, payload)
 
     # ---- Public API ----
 
@@ -116,11 +119,17 @@ class MqttClient(QObject):
         self._connected = False
 
     def publish(self, topic: str, payload: dict, qos: int = 1):
-        """线程安全的发布 — 通过 Qt 信号排队到主线程执行"""
+        """发送 JSON 消息到指定 topic"""
         if not self._client or not self._connected:
             raise ConnectionError("MQTT 未连接")
         data = json.dumps(payload, ensure_ascii=False)
-        self._publish_sig.emit(topic, data, qos)
+        print(f"[MqttClient] publish topic={topic} qos={qos}", flush=True)
+        try:
+            info = self._client.publish(topic, data, qos=qos)
+            print(f"[MqttClient] publish done mid={info.mid}", flush=True)
+        except Exception as e:
+            print(f"[MqttClient] publish EXCEPTION: {e}", flush=True)
+            raise
 
     def subscribe(self, topic: str, qos: int = 1):
         """订阅主题"""
@@ -132,15 +141,6 @@ class MqttClient(QObject):
         if self._client:
             self._client.unsubscribe(topic)
 
-    @Slot(str, str, int)
-    def _do_publish(self, topic: str, data: str, qos: int):
-        """实际的 paho publish 调用 — 始终在主线程执行"""
-        if self._client and self._connected:
-            try:
-                self._client.publish(topic, data, qos=qos)
-            except Exception as e:
-                print(f"[MqttClient] publish error: {e}")
-
     def send_command(self, dev_serial: str, cmd: str, data: Optional[dict] = None, qos: int = 1) -> int:
         """向设备发送命令，返回 msg_id"""
         from .protocol import build_request, get_topic_for_cmd