Improve Codebase Architecture Demo

Surface architectural friction and propose deepening opportunities — refactors that turn shallow modules into deep ones.

Key concepts: - Module — anything with an interface + implementation - Depth — leverage at the interface (much behaviour behind small API) - Seam — where behaviour can be altered without editing in place - Adapter — concrete thing satisfying an interface at a seam

Informed by domain language in CONTEXT.md and decisions in docs/adr/.

ROI to human

Smaller interfaces, less coupling, easier testing. Pays off every future change.