Improve Codebase Architecture

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

Glossary

Use these terms exactly. Consistent language matters — don't drift into "component," "service," "API," or "boundary." Full definitions in CONTEXT.md.

• Module — anything with an interface and implementation (function, class, package, slice)
• Interface — everything a caller must know: types, invariants, error modes, ordering, config
• Implementation — the code inside
• Depth — leverage at the interface: a lot of behaviour behind a small interface
• Shallow — interface 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
• Deep module — high functionality, simple interface, rarely changing. Wins: locality + leverage for callers

Key principle: 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.