R4: the Austin base, its action, and the base-extended bundle (#197 unit 2) - #207

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/restore@v4, actions/cache/save@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/