perf(vd960DBN): RAM 瘦身 — .bss 省 4.8KB, 栈 1.9KB→~6.7KB (2026-08-17)

栈溢出根因确认后 .bss 瘦身四刀:
- net_config.h: ETH_MAX_PACKET_SIZE 1520→768 (MSS=700最大帧758B, 省3.76KB)
- iot_mqtt_srv.c: payload[1400]→800 (loop_data最大~604B, 省600B)
- iot_mqtt_srv.h: IOT_MQTT_SEND_BUF_LEN 1024→800 (_iot_send_buf/resp/buf 同步, 省224B)
- net_srv.c: MAX_MQTTBUF_LEN 1024→800 (512仍不够勿回改, 省224B)

⚠ ETH 768 为 MSS=700 最小安全值; 若现场有大UDP包截断改回1024。
待办: .map 确认 BLE 栈占用; RECE_BUF_LEN/ARP 表可再省; 局部大数组转static。

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/net_srv.c

@@ -65,7 +65,7 @@ uint8_t MyBuf[RECE_BUF_LEN];
 
 
 #define         MAX_TMP_BUF_LEN     64
-#define         MAX_MQTTBUF_LEN     1024 //512: loop_data(4通道)≈604B > 512 → 序列化失败发全零垃圾包被broker RST (2026-07-15)
+#define         MAX_MQTTBUF_LEN     800  // 2026-08-17: 1024→800 (loop_data~604B+头<800; 512仍不够勿改回) 省224B .bss
 char mqtt_username[64] = {0};
 char mqtt_password[32] = {0};
 char mqtt_clientid[64] = {0};