Fail closed on unknown OmniVoice runtime hashes

[Blackspirits]

Summary

OmniVoice engine and voices archives are already checked against DownloadHashManager, but the integrity helper currently returns without verification when the hash key/digest cannot be resolved or when the stream is empty. That can turn a future resolver/registry mismatch into an unauthenticated unpack.

This change:

• fails closed when the OmniVoice engine/voices hash key is missing
• fails closed when the current digest is missing
• rejects empty/tampered payloads instead of treating an empty stream as verified
• rejects streams that cannot be safely read and rewound
• always rewinds the stream after hashing, including failure/cancellation paths
• preserves the existing release tag, URLs, registered digests, GGUF model-download behavior and unpack flow
• adds regressions for all seven current engine/voices digests, the real HTTP engine path, the real HTTP voices path, null/unknown keys, empty streams and non-seekable streams

Validation

• no URL or digest values were changed
• all six current engine registry hashes and the voices hash were independently compared with the sha256: asset digests published by the official niksedk/omnivoice.cpp release omnivoice-2026-08-04; all seven match
• current callers only unpack engine/voices after their download task completes successfully
• dynamic GGUF model downloads are intentionally out of scope because they do not use this pinned runtime hash registry
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34706712120

Base is upstream 9be10e12d0f04655f2f134718b00679f89b2b15f.

This PR supersedes internal draft #42.

AI assistance: ChatGPT was used to independently re-audit the OmniVoice integrity and unpack paths on current upstream, compare current registry hashes with official release asset digests, strengthen fail-closed verification, and add regression coverage.

[Blackspirits]

Independent adversarial re-check on current upstream base: reconfirmed all seven OmniVoice engine/voices registry hashes against the official niksedk/omnivoice.cpp omnivoice-2026-08-04 asset digests; null/unknown keys, empty streams, non-seekable streams and tampered payloads now fail closed, and both engine and voices public HTTP paths are covered. Full current-base CI run #34706712120 passed on the first test run with no retry. No blocker identified. Keep draft; no merge/promotion performed.