fix(ci): prevent poisoned Rust caches (#6618)

## Summary

- roll every `Swatinem/rust-cache` use back from v2.9.2 to the last
known-good v2.9.1 digest
- give Unit Tests a new `sherpa-cache-v1` key so it cannot restore the
existing poisoned artifact
- pin Renovate to v2.9.1 and add CI contracts that reject unsafe cache
actions or a misplaced generation key

## Why

After #5441 upgraded rust-cache to v2.9.2, warm-cache `main` Unit Tests
runs began failing while linking `buzz-voice` with `could not find
native static library sherpa-onnx-c-api`. The failed run at `db5617dd1`
restored the same 1.4 KB cache generation that had already failed at
`01091c15a`; the preceding cold run at `26f4c3ed3` downloaded sherpa
1.13.4 and passed.

v2.9.2 changed target cleanup, while `sherpa-onnx-sys` treats its
prebuilt `lib/` directory as proof that the native archive exists.
Rolling back the action and invalidating the affected key removes both
sides of that failure state without disabling target caching.

## Validation

At `6da0037a0407fc498cd482fcbbb74c7a15907e9f`:

- `scripts/test-rust-cache-contract.sh`
- `scripts/test-rust-cache-contract-regressions.sh`
- negative fixtures reject a bad digest in a newly named `.yaml`
workflow and a generation key moved outside the cache action's `with`
block
- YAML parse for all workflows
- release, desktop candidate, mobile release, mobile candidate, and
mobile worktree source contracts
- `just file-size-check`
- pre-commit and pre-push hooks

The PR Unit Tests run proves the cold-cache path because pull requests
restore but do not save Rust caches. The first successful `main` run
after merge will save the new Unit Tests key; the following `main` run
will exercise the warm restore.

## Related issue

None found.

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <32a2e2c9d428ee08902cab75d956da2c1d235a22d4766b0dd4138bf6e2e5db1d@buzz.block.builderlab.xyz>

renovate.json

@@ -23,6 +23,12 @@
       "separateMinorPatch": true,
       "separateMultipleMinor": false
     },
+    {
+      "description": "rust-cache v2.9.2 cleanup can preserve sherpa's prebuilt directory after removing its static archive, poisoning warm Unit Tests caches. Keep v2.9.1 until the action or sherpa validates the archive before reuse.",
+      "matchManagers": ["github-actions"],
+      "matchPackageNames": ["Swatinem/rust-cache"],
+      "allowedVersions": "<=2.9.1"
+    },
     {
       "description": "evalexpr v13 relicensed from MIT to AGPL-3.0 — pin below v13 until migrated to an MIT alternative.",
       "matchPackageNames": ["evalexpr"],