fix: generating leanSpec test assets

[shrirajpawar4]

Summary

Fixes #711.

Publishes the production fixture bundle from the prod-vectors workflow to a rolling latest GitHub release.

Changes

• Generate production fixtures with the existing just fill-ci --scheme=prod path.
• Bundle fixtures/ together with fixtures/keys/prod_scheme.
• Create release assets:
  • fixtures-prod-scheme.tar.gz
  • fixtures-prod-scheme.tar.gz.sha256
• Publish both assets to the rolling latest release.
• Replace the existing latest release/tag on each successful run.
• Update workflow permissions to contents: write.
• Add workflow concurrency to avoid racing release replacement.
• Fix stale poseidon2 cache hash paths to poseidon1.

Notes

The implementation follows the fixture artifact/release pattern used by ethereum/execution-specs, adapted to this repo’s requested rolling latest release model.

Verification

• Parsed .github/workflows/prod-vectors.yml with Ruby YAML loader.
• Ran git diff --check.
• Confirmed no remaining poseidon2 references in prod-vectors.yml.

[unnawut]

lgtm and low risk of breaking so I'll merge right away. Thanks!