Kotlin CI on GitHub Actions

Kotlin compilation is slower than Java and the daemon restarts every run.

Why it happens

kotlinc has a heavier front end, and without the configuration cache Gradle re-evaluates the whole build graph each time.

What to change

1. Turn on the Gradle configuration cache and the Kotlin incremental compiler
2. Cache ~/.gradle/caches and the build/ directories of stable modules
3. Use kotlin.incremental=true and keep the daemon warm inside the job
4. Split multiplatform targets into a matrix so they compile in parallel

Kotlin on runnerhut

jobs:
  build:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: gradle/actions/setup-gradle@v4
      - run: ./gradlew assemble --build-cache --configuration-cache

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.