Verify FFmpeg downloads before install

[Blackspirits]

Summary

FFmpeg runtime archives are unpacked by the installer flow, and the current pinned support-files release exposes SHA-256 digests for each supported platform asset. This current-base port preserves the already-reviewed integrity fix exactly.

This change:

• pins/verifies the existing Windows x64, macOS Intel and macOS ARM64 FFmpeg archive SHA-256 values
• verifies the archive before the caller can unpack it
• fails closed if a future asset URL has no pinned digest
• deletes destination-file downloads when verification fails
• keeps the verified stream rewound for the existing unpack flow
• carries the previously validated focused regressions unchanged

Current-base verification

• FfmpegDownloadService.cs is byte-for-byte identical between old review base c694526d77d92e98e5ed1449d4dbb1925a68adac and current upstream 7dbf7e03c937241ff9511ae413d5552720249de1 (blob 0f7f61014505373f1f874ea4c11601280cf9cb73)
• the previously reviewed production/test blobs were transplanted exactly onto the current upstream tree; no merge conflict or unrelated edit was introduced
• current diff is one commit / two files
• the three hashes were previously independently verified against SubtitleEdit/support-files GitHub release asset digests
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34744673250

Base is current upstream 7dbf7e03c937241ff9511ae413d5552720249de1.

This PR supersedes internal draft #24.

AI assistance: ChatGPT was used to independently verify exact base-file identity and transplant the already-reviewed FFmpeg integrity fix onto current upstream without modifying unrelated code.