Summary

• Add sccache (per-compilation-unit caching via GHA cache API) to all Linux Docker builds across build-runtime, build-node, and build-wheels workflows
• Create warm-caches.yml workflow (Bevy pattern) to pre-warm sccache on push to main and weekly cron, so subsequent builds get cache hits
• Replace actions/cache for manylinux cargo registry with sccache (which caches compilation units, not just the registry)
• Add three-layer graceful fallback so sccache failure never breaks builds:
  1. continue-on-error on sccache setup steps
  2. Conditional Docker mount (only if sccache binary exists on host)
  3. Container-level RUSTC_WRAPPER check + pass-through shim for cibuildwheel

Motivation

Three workflows independently compile the same Rust code inside manylinux Docker containers (~17-25 min each). Current caching (Swatinem + actions/cache) only caches the cargo registry — zero compilation caching inside Docker. sccache works inside containers via the GHA cache API and shares cache across all workflows.

Files Changed

Test plan

• Trigger Warm Caches via workflow_dispatch — verify cold cache (all misses in sccache --show-stats)
• Trigger Warm Caches again — verify warm cache (90%+ hits)
• Trigger Build Runtime — verify sccache hits in Docker build, macOS unaffected
• Trigger Build Node.js — verify sccache hits in Docker build
• Trigger Build Wheels — verify sccache hits in cibuildwheel container
• Verify fallback: if sccache setup step were to fail, builds should still succeed without caching