improve-codebase-architecture

# 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](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](LANGUAGE.md) for the full list): - **Deletion test**: imagine deleting the module. If complexity vanishes, it was a pass-through. If complexity reappears across N callers, it was earning its keep. - **The interface is the test surface.** - **One adapter...