C++ CI on GitHub Actions

Every translation unit recompiles even when nothing relevant changed.

Why it happens

No compiler cache is configured, so ccache or sccache cannot short-circuit identical compilations.

What to change

1. Add ccache or sccache and persist its cache directory between runs
2. Use ninja rather than make — the dependency graph is faster and more parallel
3. Cache vcpkg or Conan package directories keyed on the manifest
4. Build on a 32 vCPU runner; C++ compilation parallelises almost perfectly

C++ on runnerhut

jobs:
  build:
    runs-on: runnerhut-32vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: hendrikmuhs/ccache-action@v1
      - run: cmake -B build -G Ninja -DCMAKE_C_COMPILER_LAUNCHER=ccache
      - run: cmake --build build --parallel

The only runnerhut-specific line is `runs-on: runnerhut-32vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.