Modern React 19 platform features and rendering architecture - the React Compiler (auto-memoization) and how it changes memoization guidance, concurrent rendering (useTransition/useDeferredValue/Suspense), the use() hook, Actions (useActionState/useOptimistic/useFormStatus), ref-as-prop and Context-as-provider changes, compound components, context selectors, error boundaries, portals, virtualization, code-splitting, and the RSC/'use client' boundary.
