Phase 4: RNN warmup without init model, trajectory loss, step model export - #74

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, actions/upload-artifact@v4, tj-actions/changed-files@ed68ef82c095e0d48ec87eccea555d944a631a4c. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/