KB3b: seven-source singleton f(7) continued - parity closes L=19, beating templates closed, cover/walk dichotomy, resumable 20-cycle census (no new kappa) - #2634

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, actions/setup-python@v5, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/