Improve Codebase Architecture

Surface architectural friction and propose deepening opportunities: refactors that turn shallow modules into deep ones. The aim is testability and AI-navigability.

Glossary

Use these terms exactly in every suggestion. Full definitions are in LANGUAGE.md.

• Module: anything with an interface and an implementation.
• Interface: everything a caller must know to use the module correctly.
• Implementation: the code inside.
• Depth: leverage at the interface. Deep means high leverage; shallow means the interface is nearly as complex as the implementation.
• Seam: where an interface lives; a place behavior can be altered without editing in place.
• Adapter: a concrete thing satisfying an interface at a seam.
• Leverage: what callers get from depth.
• Locality: what maintainers get from depth.

Key principles: