proofs(agda): isolate PERMANOVA permutation bookkeeping - #59

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830, actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830, actions/cache@5a3ec84eff668545956fd18022155c47e93e2684, actions/checkout@11d5960a326750d5838078e36cf38b85af677262, julia-actions/setup-julia@4c0cb0fce8556fdb04a90347310e5db8b1f98fb9, 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/