Minimize reader load

Code is as maintainable as it is cheap to follow.

• Judge maintainability by the layers a reader must trace and the state they must hold in mind, not by line counts, complexity scores or architecture labels.
• Before adding a layer or a piece of state, check that it removes at least as much reader load elsewhere as it adds.
• Inline layers that cost more than they save: a wrapper with one caller, an adapter with no second implementation, indirection for a need that has not arrived. An adapter that hides real complexity can pass this cost test.
• Give state the narrowest scope that works: a local over a field, a field over module state, module state over a global.
• Derive a value from its source when it is needed instead of storing a copy that must be kept in sync.
• Name each invariant once, at the boundary that owns it, not in every consumer.