fix(vd960DBN): 修复 SocketSend 后重启 — RecvBuf 用错 + 栈溢出

root cause:
  1. iot_mqtt_handle_sock_int 的 CONNECT 处理调了
     WCHNET_ModifyRecvBuf(_iot_wchnet_buf), 覆盖了 WCHNET 原生的
     SocketRecvBuf, 导致后续 SocketSend 时 DMA 访问非法内存 → hard fault
  2. uint8_t tmp[RECE_BUF_LEN](1152B) 在中断栈上, 叠加调用链
     可能撑爆 2KB 栈

fix:
  1. net_srv.c WCHNET_HandleSockInt: 保留 SocketRecvBuf + KeepLive,
     其余事件委托给 iot_mqtt_handle_sock_int
  2. iot_mqtt_handle_sock_int: 移除 ModifyRecvBuf(已在 wrapper 处理)
  3. tmp[1152] 改为 static (移到 BSS, 省 1152B 栈)

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -792,13 +792,12 @@ static void iot_connect_broker(void) {
 void iot_mqtt_handle_sock_int(uint8_t socketid, uint8_t intstat) {
     if (socketid != g_iot_socket) return;
 
-    /* CONNECT 成功 — 仅标记状态，由 poll 延迟发送 CONNECT */
+    /* CONNECT 成功 — 仅标记状态, WCHNET_HandleSockInt 已设好 RecvBuf 和 KeepLive */
     if (intstat & SINT_STAT_CONNECT) {
         PRINT("IOT: TCP connected (sock=%d)\n", socketid);
-        WCHNET_ModifyRecvBuf(socketid, (uint32_t)_iot_wchnet_buf, RECE_BUF_LEN);
         g_iot_state = IOT_STATE_TCP_CONNECTED;
         _iot_recv_len = 0;
-        // 不在此发送 CONNECT，等下一轮 poll 处理
+        // 不在此发送 CONNECT，等下一轮 iot_mqtt_poll 处理
     }
 
     /* 收到数据 */
@@ -808,7 +807,7 @@ void iot_mqtt_handle_sock_int(uint8_t socketid, uint8_t intstat) {
             uint16_t space = IOT_MQTT_RECV_BUF_LEN - _iot_recv_len;
             if (recv_len > space) recv_len = space;
             uint32_t rd_len = recv_len;
-            uint8_t tmp[RECE_BUF_LEN];
+            static uint8_t tmp[RECE_BUF_LEN];  /* static: 1152B 栈分配在中断上下文中会撑爆 2KB 栈 (2026-07-23) */
             WCHNET_SocketRecv(socketid, tmp, &rd_len);
             memcpy(_iot_recv_buf + _iot_recv_len, tmp, (uint16_t)rd_len);
             _iot_recv_len += (uint16_t)rd_len;

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/net_srv.c

@@ -581,11 +581,16 @@ uint8_t i;
     // IoT MQTT mode — delegate all socket events to iot_mqtt_srv
     // (2026-07-23 修复: 旧 mqtt_connect/mqtt_data_manage 与 IoT 栈抢 socket, 致双 CONNECT → broker 踢线 → 频繁 initialize)
     if (g_sub_code_enable.iot_enable) {
-#if KEEPLIVE_ENABLE
         if (intstat & SINT_STAT_CONNECT) {
+#if KEEPLIVE_ENABLE
             WCHNET_SocketSetKeepLive(socketid, ENABLE);
-        }
 #endif
+            WCHNET_ModifyRecvBuf(socketid, (uint32_t)SocketRecvBuf[socketid], RECE_BUF_LEN);
+            PRINT("TCP Connect Success (MQTT IoT)\n");
+        }
+        if (intstat & SINT_STAT_DISCONNECT || intstat & SINT_STAT_TIM_OUT) {
+            PRINT("TCP Disconnect/Timeout (MQTT IoT)\n");
+        }
         iot_mqtt_handle_sock_int(socketid, intstat);
         return;
     }