protocol BodyRecomputable

A retained node that can re-evaluate its own body in place, using whatever view value it already holds — no new value from a parent is needed, because nothing about that value changed; only some @State/@Observable property it reads did, and that storage is a reference type living outside the struct, so it is already current. CompositeNode is the only conformer; the protocol exists so ViewInvalidation can hold a dirty node without depending on its generic view type.