Prevent stale FFmpeg loads from publishing after close or replacement

[Blackspirits]

Summary

FfmpegPlayer.LoadFile still creates its native Session inside Task.Run, while VideoPlayerControl.Open explicitly permits teardown/layout/fullscreen transitions during the await.

A Session that is still being constructed is invisible to CloseFile. It can therefore publish after close, or an older overlapping load can overwrite a newer Session.

This current-base port:

• reserves each load/close with a monotonically increasing generation;
• serializes Session publication/replacement;
• disposes stale Sessions before they can publish;
• prevents an old failed load from clearing the file name of a newer load;
• preserves current-frame cleanup and paused-first-frame behavior.

Validation

• based on upstream c77c4b428649ce7030e6cf218b4d7c384b955232
• supersedes draft Prevent stale FFmpeg loads from publishing after close or replacement #92 on the older hardware-decoder base
• current upstream still has the original unguarded async publication sequence
• no public interface change
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• UI suite: 5,151 passed, 9 skipped, 0 failed (5,160 total)
• retry step was not used
• validation run: #34778908588

AI assistance: ChatGPT was used to revalidate asynchronous Session ownership after upstream FFmpeg changes.

[Blackspirits]

Independent adversarial re-check on the revised concurrency patch: the first version still allowed an older load/close to clear a newer Session's first frame while disposing the previous Session. The final generation-safe cleanup fixes that race too: stale requests skip frame/file-name cleanup, Session publication is atomic with generation validation, and old failed loads cannot clobber newer state. Full CI #34778908588 passed restore, build and the complete suite on the first run; UITests reported 5,151 passed, 9 skipped and 0 failed. Retry was not used. No blocker identified. Keep draft; no merge performed.