docs(experiments): restate #252 and #255 on a production-faithful engine, and record why the defects were invisible - #261

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, astral-sh/setup-uv@v6. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/