OTHER: Document HMR-only unmount guards and exclude them from coverage - #1

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4.2.2, actions/setup-node@v4.4.0, actions/upload-artifact@v4.4.3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/