Tauri CI on GitHub Actions

The Rust backend recompiles fully for every platform in the matrix.

Why it happens

Cargo's target directory is not cached per platform, so each matrix leg is a cold Rust build.

What to change

1. Cache target/ and ~/.cargo per matrix leg, keyed on the platform
2. Install Linux system dependencies from a cached apt archive
3. Use native arm64 runners for Apple silicon builds rather than cross-compiling

Tauri 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: npm ci
      - run: npx tauri build

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.