refactor(vd960DBN): 去掉 heartbeat JSON 上报, MQTT PINGREQ 已保活

heartbeat 是单向设备上报, 平台不回复; MQTT PINGREQ/PINGRESP
已是双向保活。loop_data 按 g_report_cfg.interval 上报即可。

删除: iot_send_heartbeat() 函数 (省 ~40行 + BSS payload[512]→栈)

vd960DBN/BLE/OnlyUpdateApp_Peripheral/APP/iot_mqtt_srv.c

@@ -38,7 +38,7 @@ extern uint8_t SocketId_TCP;        // from net_srv.c, MQTT socket created by WC
 uint8_t  g_iot_socket = 0xFF;          // MQTT TCP socket ID
 IotMqttState g_iot_state = IOT_STATE_DISCONNECTED;
 uint8_t  g_iot_msg_id = 0;             // MQTT packet identifier
-static uint32_t _iot_last_heartbeat = 0;  // 上次心跳时刻 (ms)
+static uint32_t _iot_last_heartbeat = 0;  // 上次 PINGREQ 时刻 (ms), MQTT 保活
 static uint32_t _iot_reconnect_deadline = 0;
 static uint32_t _iot_reconnect_backoff = 0;
 static uint32_t _iot_connect_start = 0;   // TCP connect 开始时刻
@@ -814,35 +814,6 @@ void iot_mqtt_publish_sensor(void) {
     }
 }
 
-/*===========================================================================
- * 心跳上报
- *===========================================================================*/
-static void iot_send_heartbeat(void) {
-    if (g_iot_state != IOT_STATE_READY) return;
-
-    char payload[512];
-    uint8_t loop_ok[4] = {1, 1, 1, 1};
-    uint8_t i;
-    for (i = 0; i < 4; i++) {
-        /* 简化: 读取线圈状态 */
-        loop_ok[i] = 1;  // TODO: 从 Loop MCU 获取实际状态
-    }
-
-    snprintf(payload, sizeof(payload),
-        "{\"msg_id\":%lu,\"cmd\":\"heartbeat\","
-        "\"ts\":%lu,\"data\":{"
-        "\"uptime\":%lu,\"loop_status\":[%s,%s,%s,%s],"
-        "\"net_status\":true,\"iot_status\":true}}",
-        (unsigned long)(g_iot_msg_id + 1), dev_time_now(),
-        mstick() / 1000,
-        loop_ok[0] ? "true" : "false", loop_ok[1] ? "true" : "false",
-        loop_ok[2] ? "true" : "false", loop_ok[3] ? "true" : "false");
-
-    char topic[IOT_MQTT_TOPIC_MAX_LEN];
-    snprintf(topic, sizeof(topic), "dld960/%s/dev", g_iot_dev_serial);
-    iot_mqtt_publish(topic, payload, strlen(payload), 0);
-}
-
 /*===========================================================================
  * 连接管理
  *===========================================================================*/
@@ -964,7 +935,8 @@ void iot_mqtt_poll(void) {
         g_iot_state = IOT_STATE_MQTT_CONNECTING;
     }
 
-    /* MQTT Keepalive — PINGREQ */
+    /* MQTT Keepalive — PINGREQ (60s, broker 不回 → TCP 超时 → 重连)
+       loop_data 按平台下发 g_report_cfg.interval 上报, 无需额外 heartbeat JSON */
     if (g_iot_state == IOT_STATE_READY) {
         uint32_t now = mstick();
         if (now - _iot_last_heartbeat > IOT_MQTT_HEARTBEAT_MS) {
@@ -972,8 +944,6 @@ void iot_mqtt_poll(void) {
             int ping_len = MQTTSerialize_pingreq(ping_buf, sizeof(ping_buf));
             iot_mqtt_send(ping_buf, (uint16_t)ping_len);
             _iot_last_heartbeat = now;
-            // 心跳上报
-            iot_send_heartbeat();
         }
     }
 }