Zig CI on GitHub Actions

Zig's global cache is discarded, so cross-compilation targets rebuild every time.

Why it happens

~/.cache/zig holds compiled artefacts for every target triple and is empty on a fresh runner.

What to change

1. Cache ~/.cache/zig and zig-cache keyed on build.zig.zon
2. Cross-compile all targets in one job to share the cache
3. Pin the Zig version — the cache format changes between releases

Zig on runnerhut

jobs:
  build:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - uses: mlugg/setup-zig@v1
      - run: zig build test

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.