chore(ble): 增强连接与发送日志，补充 README TODO

- 连接/断开事件打印详细信息
- 发送统计计数（成功/失败）
- README 补充 L1 标定、BR 通道排查、马达频率确认待办

src/ble_transport.c

@@ -37,11 +37,15 @@ static void adv_work_handler(struct k_work *work) {
 
 /** @brief 连接建立回调，接管连接引用并收紧连接参数。 */
 static void connected(struct bt_conn *conn, uint8_t err) {
-    if (err) return;
+    if (err) {
+        LOG_WRN("Connection failed (err %u)", err);
+        return;
+    }
 
     current_conn = bt_conn_ref(conn);
     k_work_cancel_delayable(&adv_work);
     nus_notification_enabled = false;
+    LOG_INF("BLE connected");
 
     struct bt_le_conn_param param = { .interval_min = 6, .interval_max = 12, .latency = 0, .timeout = 400 };
     bt_conn_le_param_update(conn, &param);
@@ -50,7 +54,7 @@ static void connected(struct bt_conn *conn, uint8_t err) {
 /** @brief 断开回调，释放连接引用并安排延时重广播。 */
 static void disconnected(struct bt_conn *conn, uint8_t reason) {
     ARG_UNUSED(conn);
-    ARG_UNUSED(reason);
+    LOG_WRN("BLE disconnected (reason 0x%02x)", reason);
     if (current_conn) {
         bt_conn_unref(current_conn);
         current_conn = NULL;
@@ -108,23 +112,29 @@ void ble_transport_adv_start(void) {
 
 void ble_transport_send(const uint8_t *data, uint16_t len) {
     static bool first_send_logged = false;
+    static uint32_t send_ok_count;
+    static uint32_t send_fail_count;
 
     if (!nus_notification_enabled || !current_conn) return;
 
     for (int i = 0; i < 3; i++) {
         int err = bt_nus_send(current_conn, data, len);
         if (err == 0) {
+            send_ok_count++;
             if (!first_send_logged) {
                 LOG_INF("First NUS send OK (len=%u)", len);
                 first_send_logged = true;
             }
             return;
         }
+        if (i == 0) {
+            LOG_DBG("NUS send retry (err=%d)", err);
+        }
         k_usleep(500);
     }
-    LOG_WRN("NUS send failed after retries (len=%u)", len);
+    send_fail_count++;
+    LOG_WRN("NUS send failed x3 (len=%u, ok=%u, fail=%u)", len, send_ok_count, send_fail_count);
 }
-
 bool ble_transport_is_ready(void) {
     return nus_notification_enabled && (current_conn != NULL);
 }