fix(client): statsStream/statusStream 缓存为共享广播流,修延迟不回写

根因:VpnNativeBridge.statsStream getter 每次访问都 receiveBroadcastStream(),多订阅者
(连接页速度 + ConnectionController._onStats 回写 urltest 延迟)各起一条原生订阅;但
EventChannel 原生侧只留一个 sink(StatsStreamHandler.onListen 覆盖),后订阅者赢、先订阅者
变哑。速度订阅赢了 sink → _onStats 收不到 → urltest 实测(已确认到达 native:reality-out=595)
永不回写 node.ping → 连接页延迟一直 —。

修复:receiveBroadcastStream() 只调一次、缓存成 asBroadcastStream() 共享流,所有 Dart
订阅者复用同一条原生订阅。

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

client/lib/bridge/vpn_bridge.dart

@@ -223,18 +223,28 @@ class VpnNativeBridge implements VpnBridge {
   Future<void> setKillSwitch({required bool on}) =>
       _method.invokeMethod<void>('setKillSwitch', on);
 
-  @override
-  Stream<VpnStatus> get statusStream => _statusRaw
+  // ⚠️ EventChannel 原生侧只保留「一个」sink:每次 receiveBroadcastStream() + listen 都会
+  // 触发 onListen 覆盖原生 sink → 多个订阅者(连接页速度 + ConnectionController 回写延迟)
+  // 互相抢,后订阅的赢、先订阅的流变哑。必须只调一次 receiveBroadcastStream()、缓存成共享
+  // 广播流,所有 Dart 订阅者复用同一条原生订阅。
+  late final Stream<VpnStatus> _statusStream = _statusRaw
       .receiveBroadcastStream()
-      .map((e) => VpnStatus.fromString(e?.toString() ?? 'error'));
+      .map((e) => VpnStatus.fromString(e?.toString() ?? 'error'))
+      .asBroadcastStream();
 
-  @override
-  Stream<VpnStatsEvent> get statsStream => _statsRaw
+  late final Stream<VpnStatsEvent> _statsStream = _statsRaw
       .receiveBroadcastStream()
       .where((e) => e is Map)
       .map((e) => VpnStatsEvent.fromNativeMap(
             Map<Object?, Object?>.from(e as Map),
-          ));
+          ))
+      .asBroadcastStream();
+
+  @override
+  Stream<VpnStatus> get statusStream => _statusStream;
+
+  @override
+  Stream<VpnStatsEvent> get statsStream => _statsStream;
 
   @override
   void dispose() {