fix(vd960DBN): MQTT CONNECT 改为 poll 内延迟发送，避免中断内 WCHNET_SocketSend 崩溃

1. 参考 net_srv.c mqtt_connect 模式重写 iot_mqtt_send_connect:
   - clientID 用 g_dev_number_str（全局），不用 g_iot_dev_serial
   - username/password 直接赋值，不用 strlen 检查
   - memset 手动初始化，不用 brace-initializer 赋值

2. SINT_STAT_CONNECT 中断内不再调 iot_mqtt_send_connect，
   改为标记 TCP_CONNECTED，由 iot_mqtt_poll 下一轮发送 —
   参考 tcp_json_handle_sock_int 不立即发送数据的模式

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -77,33 +77,26 @@ static int iot_mqtt_send(const uint8_t *buf, uint16_t len) {
     return (ret == WCHNET_ERR_SUCCESS) ? 0 : -1;
 }
 
-/* 发送 MQTT CONNECT */
+/* 发送 MQTT CONNECT — 参考 net_srv.c mqtt_connect 实现 */
 static int iot_mqtt_send_connect(void) {
-    static MQTTPacket_connectData opts;  // static: 避免 85字节栈开销
+    static MQTTPacket_connectData opts;  // static: 避免栈开销
-    static uint8_t buf[256];             // static: 避免 256字节栈开销
+    static uint8_t buf[256];
     int len;
 
-    // struct assignment 不支持 brace initializer，用临时变量中转
+    // 手动初始化（参考 net_srv.c 模式，不依赖 brace-initializer 赋值）
-    { MQTTPacket_connectData _init = MQTTPacket_connectData_initializer; opts = _init; }
+    memset(&opts, 0, sizeof(opts));
-
+    opts.struct_id[0] = 'M'; opts.struct_id[1] = 'Q';
-    opts.MQTTVersion = 4;   // MQTT 3.1.1
+    opts.struct_id[2] = 'T'; opts.struct_id[3] = 'C';
+    opts.MQTTVersion = 4;
     opts.keepAliveInterval = IOT_MQTT_KEEPALIVE_SEC;
     opts.cleansession = 1;
 
-    // clientID
+    // clientID — 参考 net_srv.c 直接使用 g_dev_number_str
-    if (strlen((char *)iot_net_info.client_id) > 0) {
+    opts.clientID.cstring = g_dev_number_str;
-        opts.clientID.cstring = (char *)iot_net_info.client_id;
-    } else {
-        opts.clientID.cstring = g_iot_dev_serial;
-    }
 
-    // username / password
+    // username / password — 参考 net_srv.c 直接赋值
-    if (strlen((char *)iot_net_info.username) > 0) {
+    opts.username.cstring = (char *)iot_net_info.username;
-        opts.username.cstring = (char *)iot_net_info.username;
+    opts.password.cstring = (char *)iot_net_info.password;
-    }
-    if (strlen((char *)iot_net_info.password) > 0) {
-        opts.password.cstring = (char *)iot_net_info.password;
-    }
 
     len = MQTTSerialize_connect(buf, sizeof(buf), &opts);
     if (len <= 0) {
@@ -531,15 +524,13 @@ static void iot_connect_broker(void) {
 void iot_mqtt_handle_sock_int(uint8_t socketid, uint8_t intstat) {
     if (socketid != g_iot_socket) return;
 
-    /* CONNECT 成功 */
+    /* CONNECT 成功 — 仅标记状态，由 poll 延迟发送 CONNECT */
     if (intstat & SINT_STAT_CONNECT) {
         PRINT("IOT: TCP connected (sock=%d)\n", socketid);
         WCHNET_ModifyRecvBuf(socketid, (uint32_t)_iot_wchnet_buf, RECE_BUF_LEN);
         g_iot_state = IOT_STATE_TCP_CONNECTED;
         _iot_recv_len = 0;
-        // 发送 MQTT CONNECT
+        // 不在此发送 CONNECT，等下一轮 poll 处理
-        iot_mqtt_send_connect();
-        g_iot_state = IOT_STATE_MQTT_CONNECTING;
     }
 
     /* 收到数据 */
@@ -603,6 +594,12 @@ void iot_mqtt_poll(void) {
         }
     }
 
+    /* TCP 已连接 — 发送 MQTT CONNECT（延迟到 poll 而非中断内） */
+    if (g_iot_state == IOT_STATE_TCP_CONNECTED) {
+        iot_mqtt_send_connect();
+        g_iot_state = IOT_STATE_MQTT_CONNECTING;
+    }
+
     /* MQTT Keepalive — PINGREQ */
     if (g_iot_state == IOT_STATE_READY) {
         uint32_t now = mstick();