Unity CI on GitHub Actions

Unity re-imports every asset on each build, which can take longer than the build.

Why it happens

The Library folder holds the imported asset database and is discarded between runs.

What to change

1. Cache the Library folder keyed on the Unity version and asset hash
2. Use an Accelerator instance so imports are shared across machines
3. Build platforms as parallel matrix legs
4. Use a 16 vCPU runner — asset import parallelises well

Unity on runnerhut

jobs:
  build:
    runs-on: runnerhut-16vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: actions/cache@v4
        with: { path: Library, key: unity-${{ hashFiles('Assets/**') }} }
      - uses: game-ci/unity-builder@v4
        with: { targetPlatform: StandaloneLinux64 }

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.