TASK 21 slice 1 step 4: wire media into resolver + preview routing

Wire the MediaVideoSourceManager into the live pipeline (new partial
ViewModels/MainViewModel.Media.cs mirrors Webcam/Background partials):
readonly _mediaManager assigned in the core ctor (decode at 1920x1080 master,
dispatcher-coalesced preview), OnMediaPreviewBitmapChanged adopts the shared
bitmap onto every IsMediaSource with a matching MediaPath, a
Source { IsMediaSource, MediaPath } case in ResolveOutputFrame feeds
GetLatestFrame, and the manager is disposed on shutdown.

Source.DisplaySource now routes VideoImageSource for MediaSource (Source.cs),
so media previews/canvas show decoded video.

Compositor needs no change: BlitContent UniformToFill-scales any frame to the
element rect. Decoder/manager/seam derived-work mirrors ScreenCaptureManager
(citation in commit for slice-step-3).

Test (one unit for this change, mirroring the live-capture case):
Source_DisplaySource_IsVideoImageSource_WhenMediaSource. 14/14 media +
DisplaySource tests pass, build 0 warnings. Docs (TASKS/HANDOFF/ai.md) updated.

ViewModels/MainViewModel.cs

@@ -282,6 +282,14 @@ public partial class MainViewModel : ViewModelBase
         _screenCaptureManager.CaptureFailed += (key, message) =>
             AppLog.Write($"ScreenCaptureManager: capture '{key}' failed: {message}");
 
+        _mediaManager = new MediaVideoSourceManager(
+            path => new MediaVideoSource(
+                path, MediaDecodeWidth, MediaDecodeHeight,
+                new FfmpegLocator(), new FfmpegDecodeProcess()),
+            System.Windows.Application.Current?.Dispatcher);
+        _mediaManager.PreviewBitmapChanged += OnMediaPreviewBitmapChanged;
+        _mediaManager.MediaFailed += OnMediaFailed;
+
         _chatLayer = new ChatOverlayLayer(new ChatBoxRenderer());
 
         _sceneGraph = new SceneGraph();
@@ -397,6 +405,7 @@ public partial class MainViewModel : ViewModelBase
         _framePump.Dispose();
         _cameraManager.Dispose();
         _screenCaptureManager.Dispose();
+        _mediaManager.Dispose();
         _webView2Manager?.Dispose();
         _layoutStore.Dispose();
         _musicPlayer.Dispose();
@@ -452,6 +461,7 @@ public partial class MainViewModel : ViewModelBase
             Source { IsLiveCapture: true, CaptureKey: not null } live => _screenCaptureManager.GetLatestFrame(live.CaptureKey),
             Source { Type: SourceType.ChatBox } chatBox => RenderChatBox(chatBox),
             Source { Type: SourceType.WebSource } web => _webView2Manager?.GetLatestFrame(web.Id),
+            Source { IsMediaSource: true, MediaPath: not null } media => _mediaManager.GetLatestFrame(media.MediaPath),
             Source { AssetId: not null } image => StaticPixelCache.Get(image.AssetId),
             _ => null,
         };