feat: proofs of lemmas 10.12, 10.13, and 10.14 related to elementary predictable sets - #518

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