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>

app/kernel/poc/reality_client.config.json.tmpl

@@ -1,6 +1,7 @@
 {
-  "_comment": "sing-box 客户端配置模板 — VLESS+REALITY+TUN（PoC tsk_SLCsjNgtmng3）",
+  "_comment": "sing-box 客户端配置模板 — VLESS+REALITY+TUN（PoC tsk_SLCsjNgtmng3 / M4/M5 tsk_xAQhC1xuCd8x）",
   "_usage": "渲染脚本: app/kernel/poc/gen-poc-config.sh；占位符一律 __UPPER_SNAKE__",
+  "_m4_note": "URLTest 自动选线：auto-select(urltest) → 成员节点延迟最低者。手动切换经 Clash API PUT /proxies/proxy",
 
   "log": {
     "level": "info",
@@ -13,7 +14,7 @@
         "tag": "dns-remote",
         "address": "tls://1.1.1.1",
         "address_resolver": "dns-local",
-        "detour": "reality-out"
+        "detour": "proxy"
       },
       {
         "tag": "dns-local",
@@ -50,6 +51,22 @@
   ],
 
   "outbounds": [
+    {
+      "_comment": "M4: URLTest 自动选优出口组（成员：所有实际代理节点 tag）",
+      "type": "urltest",
+      "tag": "auto-select",
+      "outbounds": ["reality-out"],
+      "url": "http://www.gstatic.com/generate_204",
+      "interval": "3m",
+      "tolerance": 50
+    },
+    {
+      "_comment": "M4: Selector 出口组（Clash API 操作此组来手动覆盖 / 切回自动）",
+      "type": "selector",
+      "tag": "proxy",
+      "outbounds": ["auto-select", "reality-out"],
+      "default": "auto-select"
+    },
     {
       "type": "vless",
       "tag": "reality-out",
@@ -93,7 +110,7 @@
       { "geoip": "cn", "outbound": "direct" },
       { "geoip": "private", "outbound": "direct" }
     ],
-    "final": "reality-out",
+    "final": "proxy",
     "auto_detect_interface": true
   },