TASK 21 slice 1 step 3: IMediaFrameSource + MediaVideoSourceManager

Refactor MediaVideoSource onto IMediaFrameSource (Key/FrameAvailable/Completed/
StartAsync/StopAsync, renaming Start->StartAsync, FrameReady->FrameAvailable),
behavior preserved. Add MediaVideoSourceManager: app-wide decode-session owner
refcounted by MediaPath with Func<string,IMediaFrameSource?> factory seam,
Acquire/Release/ReleaseAll/GetLatestFrame, coalescing each file's frames onto
the UI dispatcher onto one shared WriteableBitmap; MediaFailed + PreviewBitmapChanged.

Mirrors ScreenCaptureManager (screen-capture session ownership + dispatcher
coalescing), per the codebase precedent and derivative-work rule.

Tests: MediaVideoSourceManagerTests (5 unit + 1 integration: single shared
bitmap, coalesce-to-latest); MediaVideoSourceTests updated for renames.
10/10 media tests pass, build 0 warnings. Docs (TASKS/HANDOFF/ai.md) updated.

Services/IMediaFrameSource.cs

@@ -0,0 +1,21 @@
+namespace ytLive.Services;
+
+/// <summary>
+/// TASK 21: a media file decoder exposed to <see cref="MediaVideoSourceManager"/>.
+/// Mirrors <c>IScreenCaptureSource</c>/<c>ICameraFrameSource</c> so the manager
+/// (and tests) treat a local video file like any other frame source: start/stop
+/// and a per-frame <see cref="FrameAvailable"/> event consumers coalesce.
+/// </summary>
+public interface IMediaFrameSource : IDisposable
+{
+    string Key { get; }
+
+    /// <summary>Raised (on a worker thread) for each decoded frame.</summary>
+    event Action<VideoFrame>? FrameAvailable;
+
+    /// <summary>Raised when the file finishes decoding (natural EOF).</summary>
+    event Action? Completed;
+
+    Task StartAsync(CancellationToken cancellationToken = default);
+    Task StopAsync();
+}