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/bridge/vpn_bridge_mock.dart

@@ -0,0 +1,137 @@
+// vpn_bridge_mock.dart — 假内核实现，供 UI 层先行联调
+//
+// 行为：
+//   start()  → 立即 emit connecting，[connectDelay] 后 emit on，启动统计计时器
+//   stop()   → 立即 emit off，停止统计计时器
+//   stats    → on 状态下按 [statsInterval] 周期推假数据帧
+//
+// 用法示例（UI 联调）：
+//   final bridge = VpnBridgeMock();
+//
+// 用法示例（测试）：
+//   final bridge = VpnBridgeMock(
+//     connectDelay:  Duration(milliseconds: 50),
+//     statsInterval: Duration(milliseconds: 50),
+//   );
+
+import 'dart:async';
+import 'dart:math';
+import 'vpn_bridge.dart';
+
+class VpnBridgeMock implements VpnBridge {
+  VpnBridgeMock({
+    this.connectDelay = const Duration(milliseconds: 1500),
+    this.statsInterval = const Duration(seconds: 1),
+  });
+
+  final Duration connectDelay;
+  final Duration statsInterval;
+
+  // 内部状态
+  VpnStatus _status = VpnStatus.off;
+  String _activeOutbound = 'auto';
+  bool _killSwitch = false;
+
+  final _statusCtrl = StreamController<VpnStatus>.broadcast();
+  final _statsCtrl = StreamController<VpnStatsEvent>.broadcast();
+
+  Timer? _connectTimer;
+  Timer? _statsTimer;
+
+  int _tickCount = 0;
+  final _rng = Random();
+
+  // ── helpers ───────────────────────────────────────────────────
+
+  void _emitStatus(VpnStatus s) {
+    _status = s;
+    if (!_statusCtrl.isClosed) _statusCtrl.add(s);
+  }
+
+  void _startStats() {
+    _tickCount = 0;
+    _statsTimer?.cancel();
+    _statsTimer = Timer.periodic(statsInterval, (_) {
+      _tickCount++;
+      if (!_statsCtrl.isClosed) _statsCtrl.add(_fakeTick(_tickCount));
+    });
+  }
+
+  VpnStatsEvent _fakeTick(int t) {
+    // 模拟稳定带宽 + 微抖动
+    final upSpeed = 10240.0 + _rng.nextDouble() * 2048;
+    final downSpeed = 51200.0 + _rng.nextDouble() * 8192;
+    return VpnStatsEvent(
+      uploadBytes: (upSpeed * t * statsInterval.inMilliseconds / 1000).round(),
+      downloadBytes: (downSpeed * t * statsInterval.inMilliseconds / 1000).round(),
+      uploadSpeed: upSpeed,
+      downloadSpeed: downSpeed,
+      urltestResults: const [
+        UrltestResult(tag: 'hk-01', delayMs: 18),
+        UrltestResult(tag: 'jp-01', delayMs: 32),
+        UrltestResult(tag: 'sg-01', delayMs: 54),
+      ],
+    );
+  }
+
+  // ── VpnBridge methods ─────────────────────────────────────────
+
+  @override
+  Future<void> start(String configJson) async {
+    _connectTimer?.cancel();
+    _statsTimer?.cancel();
+    _emitStatus(VpnStatus.connecting);
+    _connectTimer = Timer(connectDelay, () {
+      _emitStatus(VpnStatus.on);
+      _startStats();
+    });
+  }
+
+  @override
+  Future<void> stop() async {
+    _connectTimer?.cancel();
+    _statsTimer?.cancel();
+    _connectTimer = null;
+    _statsTimer = null;
+    _emitStatus(VpnStatus.off);
+  }
+
+  @override
+  Future<VpnStatus> getStatus() async => _status;
+
+  @override
+  Future<void> selectOutbound(String tag) async {
+    _activeOutbound = tag;
+    // 切换出口时模拟短暂重连
+    if (_status == VpnStatus.on) {
+      _statsTimer?.cancel();
+      _emitStatus(VpnStatus.connecting);
+      _connectTimer = Timer(connectDelay ~/ 2, () {
+        _emitStatus(VpnStatus.on);
+        _startStats();
+      });
+    }
+  }
+
+  @override
+  Future<String> getActiveOutbound() async => _activeOutbound;
+
+  @override
+  Future<void> setKillSwitch({required bool on}) async {
+    _killSwitch = on;
+  }
+
+  @override
+  Stream<VpnStatus> get statusStream => _statusCtrl.stream;
+
+  @override
+  Stream<VpnStatsEvent> get statsStream => _statsCtrl.stream;
+
+  @override
+  void dispose() {
+    _connectTimer?.cancel();
+    _statsTimer?.cancel();
+    _statusCtrl.close();
+    _statsCtrl.close();
+  }
+}