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. Consistent language is the point - don't drift into "component," "service," "API," or "boundary." Full definitions in LANGUAGE.md.

• Module - anything with an interface and an implementation (function, class, package, slice).
• Interface - everything a caller must know to use the module: types, invariants, error modes, ordering, config. Not just the type signature.
• Implementation - the code inside.
• Depth - leverage at the interface: a lot of behaviour behind a small interface. Deep = high leverage. Shallow = interface nearly as complex as the implementation.
• Seam - where an interface lives; a place behaviour can be altered without editing in place. (Use this, not "boundary.")
• Adapter - a concrete thing satisfying an interface at a seam.
• Leverage - what callers get from depth.
• Locality - what maintainers get from depth: change, bugs, knowledge concentrated in one place.

Key principles (see LANGUAGE.md for the full list):