Speed up Satterthwaite df with empirical (sandwich) vcov - #584

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11, actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/