Fail closed on unverified TTS voice packs

[Blackspirits]

Summary

The new TTS Voice Manager downloads catalogued voice-pack ZIPs and verifies SHA-256 when a digest is present, but the downloader currently returns successfully when the digest is empty or the downloaded stream is empty. That turns a future catalog omission into an unauthenticated install path.

This change:

• requires every voice pack to have a SHA-256 before any network request starts
• rejects streams that cannot be safely read and rewound before download
• verifies downloaded bytes against the catalog digest
• rejects empty/tampered payloads instead of treating them as successfully verified
• always rewinds the stream after hashing, including failure/cancellation paths
• updates the VoicePack contract comment to make SHA-256 mandatory
• leaves pack URLs, catalog contents, ZIP extraction and per-engine voice import behavior unchanged
• adds regressions for all eight current catalog digests, missing-digest fail-closed behavior before HTTP, tampered payload rejection/rewind, a valid SHA-256 vector, empty streams and non-seekable streams

Validation

• all seven Multilingual LibriSpeech pack hashes were independently compared with the sha256: asset digests published by SubtitleEdit/support-files release tts-voice-packs-2026-09; all seven match
• the English standard pack hash was independently compared with the voices.zip asset digest published by SubtitleEdit/support-files release qwen3-tts-cpp-2026-5; it matches
• the current installer only calls InstallPack after DownloadPack completes successfully, so integrity failure blocks extraction/import
• the reviewed production/test blobs were transplanted unchanged from the initial 7c8a audit branch onto the current review base
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34760450344
• upstream later advanced to 9c0f6bdd77bc12bafff66f748b97c8b3d3a2d5b7; the two intervening commits modify only Turkish.json and do not touch the Voice Manager/voice-pack path

Base is upstream 4771a22765acdbdf5bd5b8a1c02f378c15e73ac9.

AI assistance: ChatGPT was used to audit the newly introduced Voice Manager download/install path, verify all current catalog digests against official support-files release metadata, strengthen fail-closed verification, and add focused regression coverage.

[Blackspirits]

Independent adversarial re-check: all eight current voice-pack catalog digests were reconfirmed against the two official SubtitleEdit/support-files releases; missing SHA-256 and non-seekable streams now fail before HTTP, empty/tampered payloads fail before InstallPack, and verified streams are rewound for the existing ZIP import flow. Full CI run #34760450344 passed restore/build/full tests on the first run with no retry. No blocker identified. Keep draft; no merge/promotion performed.