merge: maestro/tsk_iP5-_Ztq5THx [桥接 API 面 Channel 契约 + 三端原生骨架] [tsk_rcNKc3GQHBUd]

合并冲突补救：原分支从 merge base 7871512 分叉，仅修改 client/ 目录，
与 main 新增的 server/(apierr/idgen/CONVENTIONS.md) 完全不重叠，
无真实文件冲突，手动应用 feature branch 的全部 client/ 改动。

client 侧新增：
- lib/bridge/vpn_bridge.dart      — Dart↔原生通道契约（冻结）
- lib/bridge/vpn_bridge_mock.dart — 假内核，UI 联调用
- lib/bridge/kernel_process.dart  — 桌面子进程管理接口
- ios/PacketTunnel/{Info.plist,PacketTunnelProvider.swift} — NEPacketTunnel 骨架
- ios/Runner/VpnManager.swift     — NETunnelProviderManager 封装
- android/.../PangolinVpnService.kt — VpnService 骨架
- android/.../VpnEventBus.kt      — Application 级状态总线
- test/bridge/vpn_bridge_mock_test.dart — 桥接层单元测试

client 侧修改：
- android/.../MainActivity.kt    — 注册三通道（MethodChannel + 2×EventChannel）
- android/.../AndroidManifest.xml — 声明 FOREGROUND_SERVICE + PangolinVpnService
- ios/Runner/AppDelegate.swift   — 注册通道 + VpnManager 初始化
- ios/Runner/Info.plist          — 新增 NSVPNUsageDescription
- ios/Runner.xcodeproj/project.pbxproj — 添加 PacketTunnel extension target
- lib/widgets/connect_button.dart — VpnStatus 迁移到 bridge/vpn_bridge.dart
- lib/widgets/home_shell.dart    — error 状态 UI 分支补全

client/lib/widgets/connect_button.dart

@@ -1,9 +1,12 @@
 // connect_button.dart — 核心连接键(三态:off / connecting / on)
+// VpnStatus 枚举定义在 lib/bridge/vpn_bridge.dart（含 error 扩展态）
 import 'package:flutter/material.dart';
+import '../bridge/vpn_bridge.dart' show VpnStatus;
 import '../pangolin_theme.dart';
 import 'pangolin_icons.dart';
 
-enum VpnStatus { off, connecting, on }
+// 重导出便于其他 widget 从 connect_button.dart 引入（向后兼容）
+export '../bridge/vpn_bridge.dart' show VpnStatus;
 
 class ConnectButton extends StatefulWidget {
   const ConnectButton({
@@ -47,12 +50,14 @@ class _ConnectButtonState extends State<ConnectButton> with SingleTickerProvider
       VpnStatus.off => c.bgSubtle,
       VpnStatus.connecting => c.accent,
       VpnStatus.on => c.success,
+      VpnStatus.error => c.danger,
     };
     final Color fg = s == VpnStatus.off ? c.accent : PangolinColors.white;
     final IconData icon = switch (s) {
       VpnStatus.off => PangolinIcons.power,
       VpnStatus.connecting => PangolinIcons.loader,
       VpnStatus.on => PangolinIcons.shieldCheck,
+      VpnStatus.error => PangolinIcons.power,
     };
 
     final List<BoxShadow> glow = s == VpnStatus.off

client/lib/widgets/home_shell.dart

@@ -117,6 +117,7 @@ class _ConnectPage extends StatelessWidget {
       VpnStatus.off => zh ? '未连接 · 轻点连接' : 'Tap to connect',
       VpnStatus.connecting => zh ? '连接中…' : 'Connecting…',
       VpnStatus.on => zh ? '已连接 · 网络已加密' : 'Connected · Encrypted',
+      VpnStatus.error => zh ? '连接错误' : 'Connection error',
     };
     return Column(children: [
       _TopBar(zh: zh, trailing: Text(status == VpnStatus.on ? (zh ? '● 在线' : '● Online') : (zh ? '○ 离线' : '○ Offline'),