fix(iot_mqtt): 精简 loop_data JSON 避免 WCHNET_SocketSend 超大包崩溃

问题: 4通道传感器数据 JSON ~700字节 → MQTT Publish 733字节
      超过 WCHNET TCP MSS(576) 和原始 MAX_MQTTBUF_LEN(512) 设计上限
      WCHNET_SocketSend(733) 导致设备重启

修复:
- 移除非必要字段: freq_current, freq_diff, condition
- 展平 misc 嵌套对象 → mt(类型) + mv(数值)
- 缩短 key 名: freq_level→fl, has_car→car, loop_ok→ok, sensitivity→sens
- 每通道 ~70字, 4通道 ~280, 全包 ~400字节
- 加安全检查: payload >450 时跳过并打印警告

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -414,17 +414,14 @@ void iot_mqtt_publish_sensor(void) {
         const char *freq_level_names[] = {"high", "mid_high", "mid_low", "low"};
 
         written = snprintf(p, remaining,
-            "%s{\"ch\":%d,\"freq_level\":\"%s\",\"has_car\":%s,"
-            "\"loop_ok\":%s,\"freq_current\":%lu,\"freq_diff\":%d,"
-            "\"sensitivity\":%d,\"condition\":%d,"
-            "\"misc\":{\"type\":\"%s\",\"value\":%lu}}",
+            "%s{\"ch\":%d,\"fl\":\"%s\",\"car\":%s,"
+            "\"ok\":%s,\"sens\":%d,\"mt\":\"%s\",\"mv\":%lu}",
             (i > 0) ? "," : "",
             i + 1,
             freq_level_names[cs->freq_level],
             cs->car_state ? "true" : "false",
             cs->loop_state ? "false" : "true",
-            cs->freq, cs->variation,
-            cs->sensitivity, cs->condition,
+            cs->sensitivity,
             misc_type_str, misc_val);
         if (written < 0 || written >= remaining) goto done;
         p += written; remaining -= written;
@@ -440,7 +437,14 @@ void iot_mqtt_publish_sensor(void) {
 
     char topic[IOT_MQTT_TOPIC_MAX_LEN];
     iot_make_topic(topic, sizeof(topic), "dev", "data", "loop");
-    iot_mqtt_publish(topic, payload, strlen(payload), 0);
+    {
+        uint16_t plen = (uint16_t)strlen(payload);
+        if (plen > 450) {
+            PRINT("IOT: payload too large (%u bytes), skipping\n", plen);
+        } else {
+            iot_mqtt_publish(topic, payload, plen, 0);
+        }
+    }
 
 done:
     InitPkgUart(&g_pkg_uart_2);