Verify Kokoro model downloads before publish

[Blackspirits]

Summary

Kokoro TTS downloads two pinned model assets directly from the koth/kokoro.cpp voices_model_files release. Their official SHA-256 digests are published upstream, but the current downloader writes bytes directly to the final paths without authenticating them first. A partial or tampered download can therefore leave a path that later existence checks treat as installed.

This change:

• pins the official SHA-256 digests for kokoro-v1.1-zh.onnx and voices-v1.1-zh.bin
• 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
• honors cancellation immediately before the verified .part is published
• leaves the Kokoro runtime archive path unchanged; runtime fail-closed hardening remains isolated in internal PR Fail closed on unknown Kokoro TTS runtime hashes #55
• adds regressions for both official digests, verified publication, tampered cleanup, preservation of an existing destination, and missing-digest fail-closed behavior

Validation

• kokoro-v1.1-zh.onnx SHA-256 independently matched the GitHub release asset digest: eefec708cbc7aba8e8129b5c2f7cb92e1fe7d281af1e1dd451592d9ff0714a0d
• voices-v1.1-zh.bin SHA-256 independently matched the GitHub release asset digest: e678019845e6cfe3b7c34531779396b28f509451b91e6535d5dc09bbf11a4be5
• 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: #34707666328

Base is upstream 9be10e12d0f04655f2f134718b00679f89b2b15f.

This PR supersedes internal draft #37.

AI assistance: ChatGPT was used to independently re-audit the Kokoro model download/publication path on current upstream, reconfirm official GitHub release digests, strengthen verified publication and cancellation behavior, port the integrity fix, and add regression coverage.

[Blackspirits]

Independent adversarial re-check: reconfirmed both Kokoro model digests against the official koth/kokoro.cpp voices_model_files release; downloads now stage to .part, verify before publish, preserve an existing destination on failed replacement, remove temporary files on failure/cancellation, and fail before HTTP if the digest is absent. Full CI run #34707666328 passed on the first test run with no retry. No blocker identified. Keep draft; no merge/promotion performed.