oneMemberToK2

Whether K2 folds two same-name same-parameter members into one, which is the only reason closing the class-side member is enough on its own. Two reference types read as a covariant override and fold; a primitive or void against anything is two Kotlin types and does not. A primitive against its own box folds too — Kotlin has one type for the pair — but nothing reaches here that way: Mixin puts the boxed member on the INTERFACE as a default, and a default is not copied onto the class, so no key collides.