fix(mqtt): 修复 loop_data>512B 溢出发垃圾包致 broker RST 重连风暴 (现场P0)

根因(非平台推测的环形缓冲/event_report队列):
  mqtt_publish 的 mqttBuf=512B < loop_data(4通道)604B
  → MQTTSerialize_publish 返回 -2 不写 buf
  → len 为 uint32_t, -2 变 42.9亿
  → WCHNET 把清零缓冲+越界相邻全局(temp_guide=report_config→report_c)
    当 520B 垃圾包发出 → broker 见非法类型0x00 RST → 重连风暴

修复(net_srv.c):
  - MAX_MQTTBUF_LEN 512→1024 (容纳 604B loop_data)
  - mqtt_publish: len uint32→int + 守卫 if(len<=0)return (序列化失败绝不发残缓冲)
  - keepalive 9→60 (报告建议, 9s过激)

event_report 协议违规修复(iot_mqtt_srv.c):
  - 重连重发保持原 msg_id/ts (V1.04 §5.3-2), 不再作废换号致平台去重失效

验证: tests/test_mqtt_publish_overflow.c 复现旧垃圾包+验证守卫/大缓冲;
      tests/test_event_report.c T8 断言重连同 msg_id. 均全过.
遗留: ts=上电秒数非Unix时间戳, 待定方案(设备无RTC/SNTP)

Refs: docs/incidents/2026-07-15-DC045A49718F-protocol-error.md

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/net_srv.c

@@ -64,7 +64,7 @@ uint8_t MyBuf[RECE_BUF_LEN];
 
 
 #define         MAX_TMP_BUF_LEN     64
-#define         MAX_MQTTBUF_LEN     512 //384     //512
+#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)
 char mqtt_username[64] = {0};
 char mqtt_password[32] = {0};
 char mqtt_clientid[64] = {0};
@@ -160,21 +160,29 @@ void MQTT_Pingreq()
 
 void mqtt_publish(char *topic, char *message, int req_qos)
 {
-    uint32_t len = 0;
+    int len = 0;                 // 必须有符号: MQTTSerialize_publish 缓冲不足返回 -2(BUFFER_TOO_SHORT)
     clear_mqtt_buf();
-    
+
     {
         PRINT("\nWill_publish_topid:%s, message:%s\n", topic, message);
     }
-    
+
     MQTTString topicString = MQTTString_initializer;
     int msglen = strlen(message);
     topicString.cstring = topic;
     static uint16_t s_mqtt_pkt_id = 0;
     uint16_t pkt_id = (req_qos > 0) ? ++s_mqtt_pkt_id : 0;
     len = MQTTSerialize_publish(mqttBuf, sizeof(mqttBuf), 0, req_qos, 0, pkt_id, topicString, (unsigned char *)message, msglen);
-//    Transport_SendPacket(mqttBuf, len);
-    WCHNET_SocketSend(SocketId_TCP, (uint8_t *)mqttBuf, &len);
+    /* 守卫: 序列化失败(缓冲不足/参数错)绝不发送残缓冲。
+       否则 len 转 uint32 成天文数字, WCHNET 把清零的 mqttBuf + 越界相邻内存当垃圾包发出,
+       broker 见非法报文类型 0x00 → RST → 重连风暴 (2026-07-15 现场事故根因) */
+    if (len <= 0) {
+        PRINT("mqtt_publish: serialize FAIL rc=%d msglen=%d buf=%d (dropped)\n",
+              len, msglen, (int)sizeof(mqttBuf));
+        return;
+    }
+    uint32_t slen = (uint32_t)len;
+    WCHNET_SocketSend(SocketId_TCP, (uint8_t *)mqttBuf, &slen);
 }
 
 void dev_initialize_pub(void)