Flutter CI on GitHub Actions

The Flutter SDK and pub cache download on every run, for every platform.

Why it happens

Neither the SDK nor ~/.pub-cache is persisted, and iOS and Android builds each pay separately.

What to change

1. Cache the Flutter SDK directory and ~/.pub-cache keyed on the Flutter version
2. Build iOS and Android as parallel matrix legs on native runners
3. Persist Pods/ and Gradle caches for the native halves
4. Run flutter test with --concurrency set to the core count

Flutter on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: subosito/flutter-action@v2
        with: { flutter-version: '3.27.0', cache: true }
      - run: flutter pub get
      - run: flutter test --concurrency 8

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.