Layers

The layers are crates. Cargo enforces the DAG through the `pkgs/mrlyrs/` manifests: core ← math ← ui and core ← font ← ui; core ← music ← apps; os leans only on core; apps sees all of those; net sees everything and owns the card. Nothing in `pkgs/` imports net; the `apps/` binaries (cli, site) sit on top and may.

The one lint

`utils/layers.py` enforces the single rule Cargo cannot: apps never import other apps. Shared app code goes in a kit, not a sibling. Run it before shipping (ship.py does).

Inside a crate

Module boundaries inside a merged crate (math/physics/crypto in mrlymath) are convention, not compiler-checked. Families keep talking through their mod.rs face.