Trait atom

A trait is a behavioral contract: a named bundle of obligations over a Self type, discharged by impl Trait for Type blocks. The boundary with the concept lattice is doctrinal: concepts say what something is (ontologically); traits say what something can do — trait hierarchies never enter the <: concept lattice, traits cannot be specialized or inherited, and classification never belongs in a trait (Modeling guidance: traits versus phases and categories). The full design is RFD 0026; the trait surface lands in stages, and what runs end-to-end is exercised by the packages in examples/.