Add synthetic fixture and Cramer-Rao validation for xray linear prediction (refs #1) - #2

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: astral-sh/setup-uv@d0cc045d04ccac9d8b7881df0226f9e82c39688e. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/