feat(client/macos): OSSystemExtensionRequest 激活 sysext

VpnChannel.activateSystemExtensionIfNeeded 实装:start 前请求系统加载/更新
PacketTunnel sysext;首启弹「隐私与安全性」待用户允许(await 阻塞到批准,
扩展没加载隧道起不来)。SysExtActivationDelegate 处理 finish/fail/replace。

至此 macOS 原生 VPN 栈构建完整:Runner 嵌入 sysext 到
Contents/Library/SystemExtensions,编译全过。运行待:sysext 签名放行 +
systemextensionsctl developer on(或 Developer ID 公证)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/macos/Runner/VpnChannel.swift

@@ -12,6 +12,7 @@
 
 import FlutterMacOS
 import NetworkExtension
+import SystemExtensions
 
 // NSLog 兼容老部署目标(Runner < macOS 11,os.Logger 不可用)。
 private func vpnLog(_ message: String) { NSLog("[pangolin/vpn] %@", message) }
@@ -24,6 +25,7 @@ final class VpnChannel: NSObject {
     private var statusObserver: NSObjectProtocol?
     private var statsTimer: Timer?
     private var manager: NETunnelProviderManager?
+    private var sysextDelegate: SysExtActivationDelegate?
 
     static func register(with registrar: FlutterPluginRegistrar) {
         let instance = VpnChannel()
@@ -100,11 +102,17 @@ final class VpnChannel: NSObject {
         return mgr
     }
 
-    // 站外 sysex 首次需用户授权激活。
+    // 请求系统加载/更新 PacketTunnel System Extension。首启系统会弹「隐私与安全性」
+    // 让用户允许;允许后 didFinishWithResult 回来。已是最新则快速完成。
     private func activateSystemExtensionIfNeeded() async throws {
-        // TODO(P1): OSSystemExtensionRequest.activationRequest(
-        //   forExtensionWithIdentifier: tunnelBundleId, queue: .main) + delegate 等待结果。
-        // 已激活则直接返回。详见安装文档步骤 4。
+        try await withCheckedThrowingContinuation { (cont: CheckedContinuation<Void, Error>) in
+            let req = OSSystemExtensionRequest.activationRequest(
+                forExtensionWithIdentifier: Self.tunnelBundleId, queue: .main)
+            let delegate = SysExtActivationDelegate(continuation: cont)
+            self.sysextDelegate = delegate  // 保活到回调结束
+            req.delegate = delegate
+            OSSystemExtensionManager.shared.submitRequest(req)
+        }
     }
 
     // ── 状态 / 速率回传 ─────────────────────────────────────────────
@@ -161,3 +169,32 @@ private final class StatsStreamHandler: NSObject, FlutterStreamHandler {
     }
     func onCancel(withArguments _: Any?) -> FlutterError? { owner?.onStatsCancel(); return nil }
 }
+
+// System Extension 激活请求回调。首启需用户在「系统设置→隐私与安全性」允许,
+// 允许后收到 didFinishWithResult(故 await 会阻塞到用户批准 —— 这是对的,
+// 扩展没加载前隧道起不来)。
+private final class SysExtActivationDelegate: NSObject, OSSystemExtensionRequestDelegate {
+    private let continuation: CheckedContinuation<Void, Error>
+    private var resumed = false
+    init(continuation: CheckedContinuation<Void, Error>) { self.continuation = continuation }
+
+    func request(_ request: OSSystemExtensionRequest,
+                 didFinishWithResult result: OSSystemExtensionRequest.Result) {
+        guard !resumed else { return }
+        resumed = true
+        continuation.resume()
+    }
+    func request(_ request: OSSystemExtensionRequest, didFailWithError error: Error) {
+        guard !resumed else { return }
+        resumed = true
+        continuation.resume(throwing: error)
+    }
+    func requestNeedsUserApproval(_ request: OSSystemExtensionRequest) {
+        vpnLog("system extension 待用户允许(系统设置 → 隐私与安全性)")
+    }
+    func request(_ request: OSSystemExtensionRequest,
+                 actionForReplacingExtension existing: OSSystemExtensionProperties,
+                 withExtension ext: OSSystemExtensionProperties) -> OSSystemExtensionRequest.ReplacementAction {
+        .replace
+    }
+}