Verify FFmpeg shared-library downloads before install

[Blackspirits]

Summary

The FFmpeg video-player shared-library downloader currently uses BtbN's mutable latest release alias and unpacks the downloaded Windows x64 archive without authenticating it.

This change:

• replaces the mutable latest URL with the dated autobuild-2026-09-11-13-20 FFmpeg 9.0 build
• pins ffmpeg-n9.0.1-29-gad500d59cb-win64-lgpl-shared-9.0.zip
• verifies its SHA-256 before the caller can unpack it
• removes partial/tampered destination files on download, cancellation, or integrity failure
• fails closed when a digest is missing
• keeps Windows x64 platform support and the existing unpack/install flow unchanged
• adds regressions for the exact published digest, removal of the mutable latest alias, tampered cleanup, valid payload acceptance, and missing-digest fail-closed behavior

Validation

• pinned SHA-256 independently matched the GitHub release asset metadata for BtbN/FFmpeg-Builds release autobuild-2026-09-11-13-20: 40eec25b2f55dcad7e4d4e640919b920d29818b56fdaf9353ce1fd8adefc9d6b
• FfmpegLibraries.MajorVersion remains 9.0, matching the pinned dated asset's release line
• BtbN's latest tag is mutable; the dated asset makes future updates explicit and the pinned digest authenticates the bytes
• no claim is made that the dated ZIP is byte-identical to any later latest alias target
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34702436873

Base is upstream 9be10e12d0f04655f2f134718b00679f89b2b15f.

This PR supersedes internal draft #39.

AI assistance: ChatGPT was used to re-audit the FFmpeg shared-library download/install path on current upstream, verify the official BtbN release asset digest, port the minimal integrity fix, and add regression coverage.