fix(vd960DBN): 心跳 msg_id 双递增修复 + %d→%lu

root cause: iot_send_heartbeat 先 ++g_iot_msg_id(JSON) 再
  iot_mqtt_publish 内部 ++g_iot_msg_id(MQTT pktid) → 跳号

fix: 心跳改用 g_iot_msg_id + 1 (与其他发布对齐)

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -817,11 +817,11 @@ static void iot_send_heartbeat(void) {
     }
 
     snprintf(payload, sizeof(payload),
-        "{\"msg_id\":%d,\"cmd\":\"heartbeat\","
+        "{\"msg_id\":%lu,\"cmd\":\"heartbeat\","
         "\"ts\":%lu,\"data\":{"
         "\"uptime\":%lu,\"loop_status\":[%s,%s,%s,%s],"
         "\"net_status\":true,\"iot_status\":true}}",
-        ++g_iot_msg_id, dev_time_now(),
+        (unsigned long)(g_iot_msg_id + 1), dev_time_now(),
         mstick() / 1000,
         loop_ok[0] ? "true" : "false", loop_ok[1] ? "true" : "false",
         loop_ok[2] ? "true" : "false", loop_ok[3] ? "true" : "false");