Fix division by zero error in prop_ppDiffR - #38

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v3, pavpanchekha/setup-z3@6b2d476d7a9227e0d8d2b94f73cd9fcba91b5e98. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/