ci: reusable rainix-rs-static workflow

[thedavidmeister]

Summary

• Reusable workflow .github/workflows/rainix-rs-static.yaml that runs rainix-rs-static (cargo fmt --check + clippy -D clippy::all) on ubuntu via rainix's rust-shell.
• Mirrors the rainix-sol-static / rainix-sol-legal pattern so consumer repos can drop their own nix-quick-install + cache plumbing.
• README entry added under the existing reusables section.

Test plan

• Self-test: existing rainix-rs-static matrix entry in test.yml continues to pass.
• After merge, migrate a consumer (rain.math.float / rain.string) to call this reusable in place of inlined nix steps and confirm CI is green.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 8a1bcfd2-18ec-46ac-91bf-fa9250ff5f49

📥 Commits

Reviewing files that changed from the base of the PR and between 5ff2f21 and fcccd3f.

📒 Files selected for processing (2)

• .github/workflows/rainix-rs-static.yaml
• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-10-rs-static-reusable

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