480ead0fc7
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.