Rust CI on GitHub Actions

Cargo rebuilds the entire dependency graph on almost every CI run.

Why it happens

target/ is not cached, or the cache key changes whenever any source file changes, so incremental state is thrown away each run.

What to change

1. Cache ~/.cargo/registry, ~/.cargo/git and target/ keyed on Cargo.lock rather than on source files
2. Use sccache so even a cold target/ reuses compiled artefacts across branches
3. Split cargo check, cargo clippy and cargo test into parallel jobs sharing one cache
4. Build release artefacts on a 16 vCPU runner — rustc scales close to linearly with codegen units

Rust on runnerhut

jobs:
  test:
    runs-on: runnerhut-16vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: actions/cache@v4
        with:
          path: |
            ~/.cargo/registry
            ~/.cargo/git
            target
          key: cargo-${{ hashFiles('**/Cargo.lock') }}
      - run: cargo test --all-features

The only runnerhut-specific line is `runs-on: runnerhut-16vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.