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/ios/PacketTunnel/Info.plist

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CFBundleDisplayName</key>
+	<string>穿山甲 Tunnel</string>
+	<key>CFBundleExecutable</key>
+	<string>$(EXECUTABLE_NAME)</string>
+	<key>CFBundleIdentifier</key>
+	<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
+	<key>CFBundleInfoDictionaryVersion</key>
+	<string>6.0</string>
+	<key>CFBundleName</key>
+	<string>PacketTunnel</string>
+	<key>CFBundlePackageType</key>
+	<string>XPC!</string>
+	<key>CFBundleShortVersionString</key>
+	<string>$(FLUTTER_BUILD_NAME)</string>
+	<key>CFBundleVersion</key>
+	<string>$(FLUTTER_BUILD_NUMBER)</string>
+	<key>NSExtension</key>
+	<dict>
+		<key>NSExtensionPointIdentifier</key>
+		<string>com.apple.networkextension.packet-tunnel</string>
+		<key>NSExtensionPrincipalClass</key>
+		<string>$(PRODUCT_MODULE_NAME).PacketTunnelProvider</string>
+	</dict>
+</dict>
+</plist>

client/ios/PacketTunnel/PacketTunnelProvider.swift

@@ -0,0 +1,78 @@
+// PacketTunnelProvider.swift — Packet Tunnel Extension 骨架
+//
+// Target: PacketTunnel (NEPacketTunnelProvider extension)
+// Bundle: com.pangolin.pangolinVpn.PacketTunnel
+//
+// 生命周期占位日志；实际 TUN 隧道建立与 libbox 接线由 11F 完成。
+//
+// 与主 App 通信：
+//   主 App → extension: NETunnelProviderSession.sendProviderMessage(_:responseHandler:)
+//   extension → 主 App: 通过 NEProvider.cancelTunnelWithError 或 NETunnelProviderSession 回调
+//
+// TODO(11F): 在 startTunnel 中初始化 sing-box/libbox TUN 模式内核。
+
+import NetworkExtension
+import os.log
+
+class PacketTunnelProvider: NEPacketTunnelProvider {
+
+    private let log = OSLog(
+        subsystem: "com.pangolin.pangolinVpn.PacketTunnel",
+        category: "VPN"
+    )
+
+    // ── NEPacketTunnelProvider 生命周期 ──────────────────────────
+
+    override func startTunnel(
+        options: [String: NSObject]?,
+        completionHandler: @escaping (Error?) -> Void
+    ) {
+        os_log(
+            "PacketTunnelProvider: startTunnel — options keys: %{public}@",
+            log: log, type: .info,
+            (options?.keys.joined(separator: ", ") ?? "(none)")
+        )
+        // TODO(11F): 解析 options["config"] → 写入临时配置文件
+        //            → 调用 libbox/sing-box TUN 启动 API
+        //            → 配置 packetFlow / setTunnelNetworkSettings
+        completionHandler(nil)
+    }
+
+    override func stopTunnel(
+        with reason: NEProviderStopReason,
+        completionHandler: @escaping () -> Void
+    ) {
+        os_log(
+            "PacketTunnelProvider: stopTunnel — reason=%d",
+            log: log, type: .info,
+            reason.rawValue
+        )
+        // TODO(11F): 停止 sing-box 内核 → 关闭 TUN fd
+        completionHandler()
+    }
+
+    override func handleAppMessage(
+        _ messageData: Data,
+        completionHandler: ((Data?) -> Void)?
+    ) {
+        os_log(
+            "PacketTunnelProvider: handleAppMessage — %d bytes",
+            log: log, type: .debug,
+            messageData.count
+        )
+        // TODO(11F): 解码 messageData（JSON 或 protobuf）→ 路由到内核命令
+        //            如 selectOutbound / getStats 等
+        completionHandler?(nil)
+    }
+
+    override func sleep(completionHandler: @escaping () -> Void) {
+        os_log("PacketTunnelProvider: sleep", log: log, type: .debug)
+        // TODO(11F): 挂起内核流量处理
+        completionHandler()
+    }
+
+    override func wake() {
+        os_log("PacketTunnelProvider: wake", log: log, type: .debug)
+        // TODO(11F): 恢复内核流量处理
+    }
+}