feat(foundations): reduction of ℤ_(p)[X]/(f) modulo p^n - #1931

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Not declared at the PR head: `NativeRecovery.val_recover`, `GaloisRingArith.reduce`. Advisory only; see the job summary.