Maven CI on GitHub Actions

Maven downloads the internet on every run.

Why it happens

~/.m2/repository is empty on a fresh runner, so every dependency and plugin is fetched again.

What to change

1. Cache ~/.m2/repository keyed on all pom.xml files
2. Build with -T 1C to use one thread per core
3. Use -o offline mode once the cache is warm to skip metadata checks
4. Add -B -ntp to cut log noise and transfer-progress overhead

Maven on runnerhut

jobs:
  build:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-java@v4
        with: { java-version: '21', distribution: temurin, cache: maven }
      - run: ./mvnw -B -ntp -T 1C verify

The only runnerhut-specific line is `runs-on: runnerhut-8vcpu-ubuntu-2404`. Everything else is standard GitHub Actions — the same actions, the same secrets and the same permissions model you use today.