Phase 0 gates: every row typed in answers-check, lint ratchet, unservable lookup rows, full offline suite in CI - #6

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. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

held-out or sealed lookup below 100%, see output