refactor: stage boundaries milestone 1 — Stages 0–2 and Stage 3's frozen interface (#1047, #1054) - #1108

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/