Validate final FFmpeg hardening stack together

[Blackspirits]

Purpose

Integration-only validation of the current FFmpeg hardening stack on upstream c77c4b428649ce7030e6cf218b4d7c384b955232.

The branch starts from #102, which already contains:

• Prevent stale FFmpeg loads from publishing after close or replacement #95: generation-safe async Session publication/cleanup;
• Keep FFmpeg partial Session teardown null-safe #98: partial-construction Session teardown safety;
• Interrupt blocked FFmpeg I/O when a load is closed or replaced #99: FFmpeg I/O cancellation through AVIOInterruptCB;
• Defer FFmpeg resource cleanup when a worker misses the stop timeout #100: deferred resource cleanup when a worker misses the stop timeout;
• Reject FFmpeg frames from cancelled sessions #102: reject frames from cancelled/stale Sessions.

It then adds the final revisions of:

• Replay from a key frame after hardware decoder send fallback #93: hardware fallback waits for a fresh replay serial and drops older queued packets/frames;
• Release detached seek frame when the FFmpeg video loop exits #94: release detached lastDropped frame on every video-loop exit;
• Fall back to software when hardware decoder open fails #96: D3D11VA → DXVA2 → software / VideoToolbox → software open fallback.

Acceptance

• full restore/build/test suite passes with all changes active together;
• no semantic or compile conflict inside FfmpegPlayer.cs;
• no changes beyond the reviewed hardening drafts.

This PR is audit evidence only and is not a merge candidate.

Result

• final integration HEAD: fba72f8d71189d10c3090dd1c9a8c50855f5d1f1
• 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: #34780685190

The final #93/#94/#95/#96/#98/#99/#100/#102 FFmpeg hardening stack is compatible when applied together.

[Blackspirits]

Final integration validation complete on the definitive stack: #93 revised replay-serial gating, #94 detached-frame cleanup, #95 generation-safe loads, #96 full hardware-open fallback chain, #98 partial-construction teardown, #99 AVIO interrupt cancellation, #100 deferred timeout cleanup, and #102 stale-frame publication rejection all coexist cleanly. Full CI #34780685190 passed restore, build and the complete suite on the first run; UITests reported 5,151 passed, 9 skipped, 0 failed. Retry was not used. This PR is audit evidence only and is not a merge candidate.