feat: 桌面端 PoC M1 — sing-box 子进程 TUN 接线 [tsk_SLCsjNgtmng3]

实现桌面端（macOS PoC）内核子进程管理，打通 sing-box TUN 模式全链路：

### client/lib/bridge/kernel_process.dart（完整实现，替换原有 stub）
- ClashApiClient: HTTP 客户端，支持 /connections / /proxies / /traffic(SSE) / /traffic(plain)
  - Bearer Token 鉴权；getConnections 用于就绪探测与流量统计；getTraffic SSE 读首帧后断开
- KernelProcess 接口: 新增 statusStream / statsStream 至接口定义
- DesktopKernelProcess:
  - spawn(configPath): 解析 Clash API 端口/secret → sudo sing-box run（macOS PoC）→
    轮询 /connections 等待就绪（20s 超时）→ emit connecting→on
  - kill(): SIGTERM + 等待 gracePeriod(5s) → SIGKILL → emit off
  - 意外退出: emit error（UI 可一键重连，不崩溃）
  - 统计轮询: 每秒 GET /connections，差分算 uploadSpeed/downloadSpeed
  - 二进制解析: ENV > exe同目录 > macOS Bundle Resources > 开发目录 > /usr/local/bin
- 辅助函数: generateClashApiPort（高位随机）、generateClashApiSecret（32B hex）

### client/lib/bridge/desktop_vpn_bridge.dart（新文件）
- DesktopVpnBridge implements VpnBridge：
  - start(configJson): injectClashApi（注入随机端口+secret）→ writeConfig（0600）→ kernel.spawn
  - stop(): kernel.kill(5s)
  - statusStream / statsStream: 代理 KernelProcess 事件流
  - selectOutbound: Clash API PUT /proxies/{group}
  - getActiveOutbound: 从 /proxies 读 now 字段
  - configDirOverride: 测试注入支持
  - injectClashApi: 静态方法，尊重已有 clash_api 配置，合并保留 experimental 其他字段

### client/test/bridge/kernel_process_test.dart（新文件）
- ClashApiClient 完整测试: headers / 解析 / PUT body / DELETE / 非 200 抛 HttpException
- generateClashApiPort / generateClashApiSecret 生成范围和格式测试
- DesktopVpnBridge.injectClashApi: 注入 / 尊重已有 / 保留字段 / 保留其他 experimental / 非法 JSON
- DesktopVpnBridge + FakeKernelProcess 集成: connecting→on / stop→off / statsStream / 意外退出 / spawn失败

### app/kernel/poc/（新目录）
- reality_client.config.json.tmpl: VLESS+REALITY+TUN 客户端配置模板
  - TUN inbound: auto_route + strict_route（macOS kill-switch 基础保护）
  - DNS: 防泄露（remote via VPN + cn 直连）
  - experimental.clash_api: 随机端口 + secret 占位符
- gen-poc-config.sh: 渲染模板为可用 JSON（从环境变量读 REALITY 参数）
- README.md: 完整 PoC 接线与 M1 验收步骤

提权说明（macOS PoC）:
  · sudo 提权（开发机需配 /etc/sudoers.d/pangolin-singbox 或有 sudo 缓存）
  · 正式版: SMJobBless Helper + 公证（BACKLOG-11D-HELPER）

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

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

@@ -0,0 +1,110 @@
+{
+  "_comment": "sing-box 客户端配置模板 — VLESS+REALITY+TUN（PoC tsk_SLCsjNgtmng3）",
+  "_usage": "渲染脚本: app/kernel/poc/gen-poc-config.sh；占位符一律 __UPPER_SNAKE__",
+
+  "log": {
+    "level": "info",
+    "timestamp": true
+  },
+
+  "dns": {
+    "servers": [
+      {
+        "tag": "dns-remote",
+        "address": "tls://1.1.1.1",
+        "address_resolver": "dns-local",
+        "detour": "reality-out"
+      },
+      {
+        "tag": "dns-local",
+        "address": "local",
+        "detour": "direct"
+      },
+      {
+        "tag": "dns-block",
+        "address": "rcode://success"
+      }
+    ],
+    "rules": [
+      { "outbound": "any", "server": "dns-local" },
+      { "geosite": "cn", "server": "dns-local" },
+      { "geoip": "private", "server": "dns-local" }
+    ],
+    "final": "dns-remote",
+    "independent_cache": true
+  },
+
+  "inbounds": [
+    {
+      "type": "tun",
+      "tag": "tun-in",
+      "inet4_address": "172.19.0.1/30",
+      "inet6_address": "fdfe:dcba:9876::1/126",
+      "mtu": 1492,
+      "auto_route": true,
+      "strict_route": true,
+      "stack": "system",
+      "sniff": true,
+      "sniff_override_destination": false
+    }
+  ],
+
+  "outbounds": [
+    {
+      "type": "vless",
+      "tag": "reality-out",
+      "server": "__SERVER_HOST__",
+      "server_port": __SERVER_PORT__,
+      "uuid": "__UUID__",
+      "flow": "xtls-rprx-vision",
+      "tls": {
+        "enabled": true,
+        "server_name": "__REALITY_SNI__",
+        "utls": {
+          "enabled": true,
+          "fingerprint": "chrome"
+        },
+        "reality": {
+          "enabled": true,
+          "public_key": "__REALITY_PUBLIC_KEY__",
+          "short_id": "__REALITY_SHORT_ID__"
+        }
+      },
+      "packet_encoding": "xudp"
+    },
+    {
+      "type": "direct",
+      "tag": "direct"
+    },
+    {
+      "type": "block",
+      "tag": "block"
+    },
+    {
+      "type": "dns",
+      "tag": "dns-out"
+    }
+  ],
+
+  "route": {
+    "rules": [
+      { "protocol": "dns", "outbound": "dns-out" },
+      { "geosite": "cn", "outbound": "direct" },
+      { "geoip": "cn", "outbound": "direct" },
+      { "geoip": "private", "outbound": "direct" }
+    ],
+    "final": "reality-out",
+    "auto_detect_interface": true
+  },
+
+  "experimental": {
+    "clash_api": {
+      "external_controller": "127.0.0.1:__CLASH_API_PORT__",
+      "secret": "__CLASH_API_SECRET__"
+    },
+    "cache_file": {
+      "enabled": true,
+      "path": "/tmp/pangolin-poc.db"
+    }
+  }
+}