Verify OmniVoice model downloads before publish

[Blackspirits]

Summary

OmniVoice downloads two pinned model files from Hugging Face resolve/main directly into their final paths. The Hugging Face repository exposes an immutable revision and SHA-256 for both files, so the client can authenticate them before publication.

This change:

• pins model downloads to immutable Hugging Face revision 017094167b5c9ed565a5076ac9b3b93c5ecf5c73
• pins SHA-256 for omnivoice-base-Q8_0.gguf
• pins SHA-256 for omnivoice-tokenizer-F32.gguf
• downloads each model to a same-directory .part file
• verifies SHA-256 before publishing the final path
• removes temporary files on download, cancellation or integrity failure
• preserves an existing destination if a replacement attempt fails
• fails closed before network access when a digest is missing
• leaves OmniVoice engine/voices runtime URLs and hash registry unchanged; runtime fail-closed hardening remains isolated in internal PR Fail closed on unknown OmniVoice runtime hashes #56
• adds regressions for immutable revision URLs, both published digests, verified publication, tampered cleanup/preservation, and missing-digest fail-closed behavior

Validation

• Hugging Face repository snapshot is pinned to 017094167b5c9ed565a5076ac9b3b93c5ecf5c73
• omnivoice-base-Q8_0.gguf SHA-256 independently matched Hugging Face/Xet metadata: 2882d887921798aea13d45236556bdf8012842ab6f8cd2690943eead6289f298
• omnivoice-tokenizer-F32.gguf SHA-256 independently matched the repository LFS/Xet pointer: 83820c6316da023076af7c1d06de5e38dcd09ae9f42203675bf8b3bd9a58e330
• no runtime URL/hash behavior is changed by this PR
• full solution restore passed
• full solution build passed
• full solution test suite passed on the first run
• retry step was not used
• validation run: #34720265343

Base is current upstream fa6d8623c2adb0c646b6d31a11b221b855339bbe.

AI assistance: ChatGPT was used to audit the OmniVoice model provenance/publication path, verify immutable Hugging Face revision and published file digests, prepare the minimal pin/hash/atomic-publication fix, and add regression coverage.

[Blackspirits]

Independent adversarial re-check: the official Hugging Face repository currently exposes verified revision 0170941, whose link resolves to the pinned full revision 017094167b5c9ed565a5076ac9b3b93c5ecf5c73; omnivoice-base-Q8_0.gguf publishes SHA-256 2882d887921798aea13d45236556bdf8012842ab6f8cd2690943eead6289f298, and the tokenizer LFS pointer publishes 83820c6316da023076af7c1d06de5e38dcd09ae9f42203675bf8b3bd9a58e330. The diff stages to .part, verifies before publish, preserves existing files on failure, and fails before HTTP on missing digest. Full CI run #34720265343 passed on the first test run with no retry. No blocker identified. Keep draft; no merge/promotion performed.