Fix LLC center-mask fallback and fixture schema - #820

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@6f8efc29b200d32929f49075959781ed54ec270c, actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065, astral-sh/setup-uv@d0d8abe699bfb85fec6de9f7adb5ae17292296ff. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/