Validate current FFmpeg hardening together

[Blackspirits]

Purpose

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

The branch starts from the final #95 HEAD and adds:

• Replay from a key frame after hardware decoder send fallback #93: replay from a key frame after hardware avcodec_send_packet fallback;
• 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: fall back to software when hardware avcodec_open2 fails.

#95 already contributes generation-safe asynchronous Session publication/cleanup.

Acceptance

• full restore/build/test suite must pass with all four changes active together;
• no conflicts in FfmpegPlayer.cs;
• no production changes beyond the four already reviewed drafts.

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

Result

• 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: #34779155945

All four current FFmpeg hardening drafts are compatible when applied together.

[Blackspirits]

Integration validation complete: #93, #94, #95 and #96 are compatible when active together on c77c4b4. Full CI #34779155945 passed restore, build and the complete solution 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.