Verify libmpv downloads before install

[Blackspirits]

Summary

The Windows libmpv runtime downloader fetches pinned support-files ZIP archives that the UI unpacks immediately after the task completes. This current-base port preserves the already-reviewed checksum verification exactly.

This change:

• pins/verifies the current x64 and ARM64 libmpv archive SHA-256 values
• verifies the archive before a successful download task can return to the unpacking UI
• fails closed if a future asset URL has no pinned digest
• deletes a destination-file download when verification fails
• keeps verified/rejected streams rewound for existing callers
• carries the previously validated focused regressions unchanged

Current-base verification

• LibMpvDownloadService .cs is byte-for-byte identical between old review base c694526d77d92e98e5ed1449d4dbb1925a68adac and current upstream 7dbf7e03c937241ff9511ae413d5552720249de1 (blob bc919ed6ce2d0b839b51e11ee51c0ae6b20e3913)
• 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
• both hashes were previously independently verified against SubtitleEdit/support-files release libmpv-2026-08-14b
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34744731964

Base is current upstream 7dbf7e03c937241ff9511ae413d5552720249de1.

This PR supersedes internal draft #25.

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