feat(app): 完善播放协议搜索与空状态

lib/pages/player/widgets/player_video_surface.dart

@@ -10,16 +10,18 @@ const Duration _videoFadeDuration = Duration(milliseconds: 200);
 const double _errorHorizontal = 32;
 const int _errorMaxLines = 3;
 
-/// 黑色底层与视频层共用同一控制器，视频首帧就绪后再淡入。
+/// 黑色底层与视频层共用同一控制器。清晰度切换时用仍在播放的旧画面盖住新纹理的黑帧。
 class PlayerVideoSurface extends StatefulWidget {
   const PlayerVideoSurface({
     super.key,
     required this.controller,
     required this.videoUrl,
     required this.theater,
+    this.handoff,
   });
 
   final VideoPlayerController? controller;
+  final VideoPlayerController? handoff;
   final String videoUrl;
   final bool theater;
 
@@ -48,7 +50,8 @@ class _PlayerVideoSurfaceState extends State<PlayerVideoSurface> {
 
   Widget _frame(VideoPlayerValue? value) {
     final controller = widget.controller;
-    final reveal = _shouldReveal(value);
+    final handoff = _liveHandoff(controller);
+    final reveal = _shouldReveal(value, handoff != null);
     _logStateChange(value);
     return Stack(
       fit: StackFit.expand,
@@ -60,7 +63,7 @@ class _PlayerVideoSurfaceState extends State<PlayerVideoSurface> {
         AnimatedOpacity(
           key: const Key('player-video-opacity'),
           opacity: reveal ? 1 : 0,
-          duration: _videoFadeDuration,
+          duration: handoff != null ? Duration.zero : _videoFadeDuration,
           curve: Curves.easeOut,
           child: reveal && controller != null
               ? _VideoLayer(
@@ -70,6 +73,13 @@ class _PlayerVideoSurfaceState extends State<PlayerVideoSurface> {
                 )
               : const SizedBox.expand(),
         ),
+        if (handoff != null)
+          _VideoLayer(
+            key: const Key('player-video-handoff'),
+            controller: handoff,
+            value: handoff.value,
+            theater: widget.theater,
+          ),
         if (kDebugMode && value?.hasError == true)
           Center(
             child: Padding(
@@ -88,12 +98,18 @@ class _PlayerVideoSurfaceState extends State<PlayerVideoSurface> {
     );
   }
 
-  bool _shouldReveal(VideoPlayerValue? value) {
-    return value != null &&
-        !value.hasError &&
-        value.isInitialized &&
-        value.size.width > 0 &&
-        (value.position > Duration.zero || value.isPlaying);
+  bool _shouldReveal(VideoPlayerValue? value, bool covered) {
+    if (value == null || value.hasError || !value.isInitialized) return false;
+    if (value.size.width <= 0) return false;
+    if (covered) return true;
+    return value.position > Duration.zero || value.isPlaying;
+  }
+
+  /// 只画会话仍持有的旧画面，控制器一旦被换掉就不再引用。
+  VideoPlayerController? _liveHandoff(VideoPlayerController? controller) {
+    final handoff = widget.handoff;
+    if (handoff == null || identical(handoff, controller)) return null;
+    return handoff;
   }
 
   void _logStateChange(VideoPlayerValue? value) {
@@ -135,6 +151,7 @@ class _PlayerVideoSurfaceState extends State<PlayerVideoSurface> {
 
 class _VideoLayer extends StatelessWidget {
   const _VideoLayer({
+    super.key,
     required this.controller,
     required this.value,
     required this.theater,