feat(M4/M5): URLTest 自动选线 + Kill-switch + 弹性重连 [tsk_xAQhC1xuCd8x]

M4 URLTest 自动选线：
- ClashApiClient.getGroupDelay() — GET /group/<name>/delay 触发按需测速
- ClashApiClient.extractUrltestResults() — 从 /proxies 响应解析 URLTest
  组成员最新延迟，填充 stats 帧的 urltestResults 字段
- DesktopKernelProcess._startStatsPoll() — 每秒并行拉取连接统计 + URLTest
  延迟（_clashApi.getProxies()），URLTest 失败不影响主统计
- DesktopVpnBridge.selectOutbound(tag) — Clash API PUT /proxies/proxy，
  tag 可传节点 tag 或 urltest 组 tag（如 "auto-select"）恢复自动
- DesktopVpnBridge.getActiveOutbound() — GET /proxies 读 proxy.now
- app/kernel/poc/reality_client.config.json.tmpl — 增加 urltest("auto-select")
  + selector("proxy") 出口组，route.final 改为 "proxy"

M5 Kill-switch + 弹性重连：
- DesktopVpnBridge.setKillSwitch({required bool on}) — 将 _killSwitchEnabled
  写入 TUN inbound strict_route 字段（applyKillSwitchToConfig）；内核运行中
  触发静默重载（kill → start(lastConfig)）
- DesktopVpnBridge.applyKillSwitchToConfig() — static，killSwitch=true ↔
  TUN strict_route=true，killSwitch=false ↔ strict_route=false
- 自动退避重连：kernel 崩溃推 error → _scheduleReconnect() 退避序列
  1s/2s/4s/8s/16s/30s（上限），stop() 取消，重连成功后重置计数器
- DesktopKernelProcess 意外退出清理 _clashApi（避免下次 spawn 残留）
- app/pangolin/test/killswitch_checklist.md — 三端故障注入验收清单
  （Desktop ✅，iOS/Android 欠账说明）

新增测试（client/test/bridge/desktop_vpn_bridge_m4m5_test.dart）：
- M4: extractUrltestResults 解析/空/无 history 边界
- M4: getGroupDelay HTTP 请求格式验证
- M4: selectOutbound / getActiveOutbound Clash API 调用断言
- M5: applyKillSwitchToConfig 字段覆盖 + 无 TUN 边界
- M5: setKillSwitch 状态注入 + 同值幂等
- M5: 崩溃→error 不崩 / 首次重连在退避延迟内触发 / stop 后不再重连

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/bridge/kernel_process.dart

@@ -167,6 +167,70 @@ class ClashApiClient {
     }
   }
 
+  // ── GET /group/<name>/delay ───────────────────────────────────
+  // 触发一次 URLTest 延迟测试并返回结果 {tag: delayMs}。
+  // 注意：此接口会发起真实网络探测，耗时约 timeoutMs。
+  // 若只需读缓存延迟，使用 getProxies() + extractUrltestResults()。
+  Future<Map<String, int>> getGroupDelay(
+    String groupName, {
+    String testUrl = 'http://www.gstatic.com/generate_204',
+    int timeoutMs = 3000,
+  }) async {
+    final uri = Uri.parse('$baseUrl/group/$groupName/delay').replace(
+      queryParameters: {
+        'url': testUrl,
+        'timeout': timeoutMs.toString(),
+      },
+    );
+    final response = await _http
+        .get(uri, headers: _headers)
+        .timeout(Duration(milliseconds: timeoutMs + 2000));
+    if (response.statusCode != 200) {
+      throw HttpException('getGroupDelay($groupName): ${response.statusCode}');
+    }
+    final data = jsonDecode(response.body);
+    if (data is! Map) return {};
+    return data.cast<String, int>();
+  }
+
+  // ── 工具：从 /proxies 响应提取 URLTest 延迟列表 ──────────────
+  // 扫描所有类型为 URLTest 的出口组，提取成员节点的最新 history delay。
+  static List<UrltestResult> extractUrltestResults(
+      Map<String, dynamic> proxiesResponse) {
+    final proxies = proxiesResponse['proxies'];
+    if (proxies is! Map) return const [];
+
+    final results = <UrltestResult>[];
+
+    for (final entry in proxies.entries) {
+      final p = entry.value;
+      if (p is! Map) continue;
+      final type = (p['type'] as String?) ?? '';
+      // URLTest / Fallback 组都有 all 成员列表
+      if (type != 'URLTest' && type != 'Fallback') continue;
+
+      final all = p['all'];
+      if (all is! List) continue;
+
+      for (final memberTag in all) {
+        if (memberTag is! String) continue;
+        final member = proxies[memberTag];
+        if (member is! Map) continue;
+
+        final history = member['history'];
+        if (history is List && history.isNotEmpty) {
+          final last = history.last;
+          if (last is Map) {
+            final delay = (last['delay'] as num?)?.toInt() ?? -1;
+            results.add(UrltestResult(tag: memberTag, delayMs: delay));
+          }
+        }
+      }
+    }
+
+    return results;
+  }
+
   void dispose() => _http.close();
 }
 
@@ -243,6 +307,9 @@ class DesktopKernelProcess implements KernelProcess {
   int _prevUpTotal = 0;
   DateTime _prevPollTime = DateTime.now();
 
+  // 最新 URLTest 延迟列表（每次 stats 轮询时更新）
+  List<UrltestResult> _lastUrltestResults = const [];
+
   final _statusCtrl = StreamController<VpnStatus>.broadcast();
   final _logCtrl = StreamController<String>.broadcast();
   final _statsCtrl = StreamController<VpnStatsEvent>.broadcast();
@@ -327,6 +394,9 @@ class DesktopKernelProcess implements KernelProcess {
         _running = false;
         _statsTimer?.cancel();
         _statsTimer = null;
+        // 清理 Clash API 客户端，确保下次 spawn() 重建干净的客户端
+        _clashApi?.dispose();
+        _clashApi = null;
         _log('[kernel] unexpected exit: code=$code');
         _emitStatus(VpnStatus.error);
       }
@@ -406,6 +476,7 @@ class DesktopKernelProcess implements KernelProcess {
     _prevDownTotal = 0;
     _prevUpTotal = 0;
     _prevPollTime = DateTime.now();
+    _lastUrltestResults = const [];
   }
 
   // ── 就绪等待 ─────────────────────────────────────────────────
@@ -435,7 +506,17 @@ class DesktopKernelProcess implements KernelProcess {
       if (!_running || _statsCtrl.isClosed) return;
       try {
         final now = DateTime.now();
-        final data = await _clashApi!.getConnections();
+
+        // 并行拉取连接统计与 URLTest 延迟（Best-effort：URLTest 失败不影响主统计）
+        final connFuture = _clashApi!.getConnections();
+        final proxiesFuture = _clashApi!.getProxies().then((p) {
+          _lastUrltestResults = ClashApiClient.extractUrltestResults(p);
+        }).catchError((_) {
+          // 读取失败静默跳过，保留上次缓存值
+        });
+
+        final data = await connFuture;
+        await proxiesFuture; // 等 URLTest 解析完再组帧
 
         final downTotal =
             (data['downloadTotal'] as num?)?.toInt() ?? _prevDownTotal;
@@ -462,7 +543,7 @@ class DesktopKernelProcess implements KernelProcess {
             downloadBytes: downTotal,
             uploadSpeed: upSpeed.clamp(0, double.infinity),
             downloadSpeed: downSpeed.clamp(0, double.infinity),
-            urltestResults: const [], // URLTest 节点选优归 11G
+            urltestResults: _lastUrltestResults,
           ));
         }
       } catch (_) {