Vitest CI on GitHub Actions

Vitest re-optimises dependencies and runs in a single thread pool.

Why it happens

node_modules/.vite is discarded and the default pool configuration is conservative.

What to change

1. Cache node_modules/.vite keyed on the lockfile
2. Use --pool=threads with maxThreads set to the core count
3. Shard with --shard for large suites

Vitest on runnerhut

jobs:
  test:
    runs-on: runnerhut-8vcpu-ubuntu-2404
    steps:
      - uses: actions/checkout@v4
      - run: npm ci
      - run: npx vitest run --pool=threads --reporter=dot

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.