fix(vd960DBN): IoT MQTT 栈独立接管 socket 事件, 消除双 CONNECT 致频繁 initialize

root cause: WCHNET_HandleSockInt 在 iot_enable=1 时仍走旧 MQTT 代码:
  CONNECT → mqtt_connect()     ← 发 MQTT CONNECT #1
  iot_mqtt_poll() → send_connect() ← 发 MQTT CONNECT #2 (同一socket)
  broker 见双 CONNECT → 协议违规踢线 → 4s 后重连 → 循环
  RECV → mqtt_data_manage()   ← 旧代码收 CONNACK
        → dev_initialize_pub() ← initialize msg_id=1, 强设 g_iot_state=READY

fix:
  1. net_srv.c WCHNET_HandleSockInt: iot_enable 时直接调 iot_mqtt_handle_sock_int
  2. peripheral_main.c: poll_mqtt() → iot_mqtt_poll() (IoT 自有 PINGREQ)

BSS savings: -1024B (data_json eliminated), send buffer 隔离 (v3)

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/net_srv.c

@@ -578,28 +578,15 @@ uint8_t i;
         return;
     }
     
-    // IoT MQTT mode — inline MQTT handling (ref: DBN101GA)
+    // IoT MQTT mode — delegate all socket events to iot_mqtt_srv
+    // (2026-07-23 修复: 旧 mqtt_connect/mqtt_data_manage 与 IoT 栈抢 socket, 致双 CONNECT → broker 踢线 → 频繁 initialize)
     if (g_sub_code_enable.iot_enable) {
-        if (intstat & SINT_STAT_RECV) {
-            mqtt_data_manage(socketid);
-        }
-        if (intstat & SINT_STAT_CONNECT) {
 #if KEEPLIVE_ENABLE
+        if (intstat & SINT_STAT_CONNECT) {
             WCHNET_SocketSetKeepLive(socketid, ENABLE);
+        }
 #endif
-            WCHNET_ModifyRecvBuf(socketid, (uint32_t)SocketRecvBuf[socketid], RECE_BUF_LEN);
-            PRINT("TCP Connect Success (MQTT)\n");
-            mqtt_connect();
-        }
-        if (intstat & SINT_STAT_DISCONNECT) {
-            PRINT("TCP Disconnect (MQTT)\n");
-            g_iot_state = IOT_STATE_DISCONNECTED;
-            g_net_state.flag = 1;
-        }
-        if (intstat & SINT_STAT_TIM_OUT) {
-            PRINT("TCP Timeout (MQTT)\n");
-            g_net_state.flag = 1;
-        }
+        iot_mqtt_handle_sock_int(socketid, intstat);
         return;
     }