[krishicks]

Summary

The Actions cache is at its 50 GB quota and turns over within hours, so main's caches get evicted. Most of the space is per-binary release build caches saved from PR branches, plus rust-checks caches that every PR saves because main never writes one. This PR saves per-binary caches only on main and adds a workflow that keeps main's rust-checks cache warm.

Related Issue

None. This is a CI build fix from an author with write access.

Changes

• build-rust-binary saves its rust-binaries-v2-* cache only on refs/heads/main. Other branches still restore main's. Two PRs alone held about 10 GiB each of these caches.
• New .github/workflows/rust-cache.yml runs on pushes to main. It builds the Branch Checks rust-test targets with --no-run on the same three runners, with the same env and cache key. Branch Checks only triggers on pull-request/N, merge_group and dispatch, so main previously had no rust-checks cache for PRs to restore.
• setup-rust exposes cache-hit, so the warming job skips the build when main's cache already matches.
• PR branches still save rust-checks. Swatinem/rust-cache skips saving on an exact key hit, so once main is warm, PRs whose dependencies match main no longer write duplicates. PRs that change dependencies or the Nix dev shell keep their own warm cache across pushes.

Testing

• actionlint passes on the changed files. The only warnings are the existing self-hosted runner labels.
• After merge: confirm Rust Cache saves v0-rust-rust-checks-v2-* entries for refs/heads/main, and that PR rust-test jobs restore them.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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