- Add .cargo/config.toml with rustc-wrapper = sccache for local caching
- Install sccache in CI clippy/test jobs via prebuilt musl binary
- Add actions/cache to persist sccache dir across CI runs (keyed by Cargo.lock)
- Add BuildKit cache mounts in Dockerfile for sccache across Docker builds
- Disable sccache for fmt/audit jobs where it adds no value
Local clean-rebuild speedup: 1m58s -> 34s (3.4x) with warm cache.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
