feat(client/macos): 主 app VPN 接线编译通过 — VpnChannel 入 Runner target

- VpnChannel.swift 加入 Runner target(Compile Sources)
- Logger→NSLog(Runner 部署目标 10.15,os.Logger 需 11)+ 去掉未用的
  SystemExtensions import
- 至此原生 VPN 控制链路全编译通过:主 app(NETunnelProviderManager 启停/
  状态)+ 扩展(libbox)。kUseNativeVpnMacOS 仍 false(待隧道能跑切换)。

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

client/macos/Runner/VpnChannel.swift

@@ -12,10 +12,9 @@
 
 import FlutterMacOS
 import NetworkExtension
-import SystemExtensions
-import os
 
-private let log = Logger(subsystem: "com.pangolin.pangolin", category: "vpn")
+// 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) }
 
 final class VpnChannel: NSObject {
     private static let tunnelBundleId = "com.pangolin.pangolin.PacketTunnel"
@@ -76,7 +75,7 @@ final class VpnChannel: NSObject {
             ])
             result(nil)
         } catch {
-            log.error("start failed: \(error.localizedDescription)")
+            vpnLog("start failed: \(error.localizedDescription)")
             result(FlutterError(code: "start_failed", message: error.localizedDescription, details: nil))
         }
     }