refactor(vd960DBN): 缓冲区 2048→1024，单次交互不超 1KB

- IOT_MQTT_RECV_BUF_LEN: 2048 → 1024
- IOT_MQTT_SEND_BUF_LEN: 2048 → 1024
- data_json 改用宏
- iot_handle_publish 的 json[] 改 static 防栈溢出

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -168,7 +168,7 @@ static int iot_mqtt_publish(const char *topic, const char *payload,
 
 /* 处理一条收到的 MQTT PUBLISH 消息 */
 static void iot_handle_publish(const char *topic, uint8_t *payload, int payload_len) {
-    char json[IOT_MQTT_RECV_BUF_LEN];
+    static char json[IOT_MQTT_RECV_BUF_LEN];  // static: 避免 1KB 栈开销
     int copy_len = payload_len < (int)sizeof(json) - 1 ? payload_len : (int)sizeof(json) - 1;
     memcpy(json, payload, copy_len);
     json[copy_len] = '\0';
@@ -392,7 +392,7 @@ void iot_mqtt_publish_sensor(void) {
     }
 
     /* 构建 loop_data JSON */
-    static char data_json[2048];   // static: 避免 2KB 栈溢出
+    static char data_json[IOT_MQTT_SEND_BUF_LEN];   // static: 避免栈溢出
     char *p = data_json;
     int remaining = sizeof(data_json);
     int written;