improve-architecture

Iteration loop: explore for friction, present deepening candidates, grill the chosen one, update domain artifacts inline. Vocabulary is load-bearing. See `references/LANGUAGE.md`. ## Vocabulary [LOAD-BEARING] Use these terms exactly. Do not substitute "component," "service," "API," or "boundary." Full definitions in `references/LANGUAGE.md`. - **Module**: anything with an interface and an implementation (function, class, package, slice). Scale-agnostic. - **Interface**: every fact a caller must know: types, invariants, ordering, error modes, config, performance shape. Not just signature. - **Depth**: leverage at the interface. Deep = much behaviour behind a small interface. Shallow = interface as complex as implementation. - **Seam** (Feathers): 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. Role, not substance. - **Leverage**: capability callers gain per unit of interface learned. - **Locality**: concentration of change, bug, and knowledge at one site for maintainers. ## Principles - **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. - **Interface = test surface.** Tests cross the same seam callers cross. Wanting to test past the interface = wrong shape. - **One adapter = hypothetical seam. Two adapters = real seam.** No port withou...