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/peripheral_main.c

@@ -271,7 +271,7 @@ void Main_Circulation(void)
 
         if (g_sub_code_enable.iot_enable) {
             iot_mqtt_publish_sensor();  // Push 0xC0 sensor data to MQTT broker
-            poll_mqtt();                // MQTT PINGREQ heartbeat (in net_srv.c)
+            iot_mqtt_poll();            // IoT MQTT state machine + PINGREQ keepalive
         } else {
             tcp_json_push_sensor();     // Push 0xC0 sensor data to TCP JSON client
             tcp_json_poll();