Add a Lean model of the CCL type system and fuzz the solver against it - #79

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5, dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36, Swatinem/rust-cache@779680da715d629ac1d338a641029a2f4372abb5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/