perf: rework the carré du champ inner loop; retire the porting plan - #26

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684, actions/checkout@v4, julia-actions/setup-julia@v2, pyTooling/Actions/with-post-step@42e17fae05f224e5ac3d79d021a4e3577878efe5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/