Kotlin Multiplatform CI on GitHub Actions

Each target compiles serially and the Kotlin/Native cache is discarded.

Why it happens

~/.konan holds the Kotlin/Native toolchain and compiled artefacts and is cold on every run.

What to change

1. Cache ~/.konan and ~/.gradle/caches keyed on the Kotlin version
2. Compile JVM, JS and native targets as parallel matrix legs
3. Build Apple targets on a macOS runner and everything else on Linux

Kotlin Multiplatform on runnerhut

jobs:
  build:
    strategy:
      matrix:
        os: [runnerhut-8vcpu-ubuntu-2404, runnerhut-macos-15-m4]
    runs-on: ${{ matrix.os }}
    steps:
      - uses: actions/checkout@v4
      - run: ./gradlew build --build-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.