Refs passed through context poison every ctx read for the React Compiler: share elements through callback refs

the shared elements live in a small registry: each part registers with a stable callback ref, and the registry is read only in effects and handlers, never while rendering. One-shot hand-offs, such as where focus goes next, are methods on that object, not properties to assign. React 19's callback-ref cleanup removes an element on unmount, and only if it is still the registered one, so a copy that is animating out never unregisters the one that replaced it.