feat: IoT MQTT 客户端实现 — NET_SSC_ENABLE=0 时启用

新增文件:
- iot_mqtt_srv.h/c: MQTT 客户端 (TCP→CONNECT→SUBSCRIBE→READY)
  支持: dev_info_query, pwd_verify (其他命令框架预留)
  上报: loop_data (传感器), heartbeat (心跳 60s)
  断线重连: 指数退避 5s~60s
  协议: DLD960 IoT MQTT V1.00 (dld960/{sn}/...)

修改:
- net_config.h: 新增 NET_IOT_ENABLE = !NET_SSC_ENABLE,
  IoT 模式分配 1 TCP socket (MQTT client)
- net_srv.c: IoT 初始化 + socket 中断路由
- peripheral_main.c: IoT poll + sensor publish 调用

模式:
  NET_SSC_ENABLE=1 → SCC + TCP JSON Server (原有行为)
  NET_SSC_ENABLE=0 → IoT MQTT Client (新增)

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/include/net_config.h

@@ -19,9 +19,14 @@ extern "C" {
 
 /*********************************************************************
  * Feature toggles (0 = disabled, 1 = enabled)
+ *
+ * 模式互斥:
+ *   NET_SSC_ENABLE=1 → SSC 模式 (UDP+TCP client) + TCP JSON server
+ *   NET_SSC_ENABLE=0 → IoT MQTT 模式 (MQTT client)
  */
 #define NET_SSC_ENABLE                0   /* SSC (UDP + TCP client) protocol support */
 #define NET_JSON_ENABLE               1   /* TCP JSON protocol server (port 5960) */
+#define NET_IOT_ENABLE                (!NET_SSC_ENABLE)  /* IoT MQTT client (auto-derived) */
 
 /*********************************************************************
  * socket configuration, IPRAW + UDP + TCP + TCP_LISTEN = number of sockets
@@ -29,17 +34,19 @@ extern "C" {
 #define WCHNET_NUM_IPRAW              0  /* Number of IPRAW connections */
 
 #if NET_SSC_ENABLE
-#define WCHNET_NUM_UDP                1  /* The number of UDP connections */
+#define WCHNET_NUM_UDP                1  /* SSC mode: 1 UDP */
 #else
-#define WCHNET_NUM_UDP                0
+#define WCHNET_NUM_UDP                0  /* IoT mode: 0 UDP */
 #endif
 
 #if NET_JSON_ENABLE && NET_SSC_ENABLE
-#define WCHNET_NUM_TCP                2  /* JSON listen+data(1) + SSC client(1) */
+#define WCHNET_NUM_TCP                2  /* SSC+JSON: JSON(1) + SSC client(1) */
 #elif NET_JSON_ENABLE
-#define WCHNET_NUM_TCP                1  /* JSON listen+data(1) */
+#define WCHNET_NUM_TCP                1  /* JSON only: listen+data(1) */
 #elif NET_SSC_ENABLE
-#define WCHNET_NUM_TCP                1  /* SSC client(1) */
+#define WCHNET_NUM_TCP                1  /* SSC only: TCP client(1) */
+#elif NET_IOT_ENABLE
+#define WCHNET_NUM_TCP                1  /* IoT MQTT client(1) */
 #else
 #define WCHNET_NUM_TCP                0
 #endif