app-structure

Defines the App Router architecture for an ultraweb build — React Server Components by default, "use client" pushed to leaf components and never into layouts or pages, server/client composition via children-as-slots, a decision table for where every kind of state lives (server, URL, client leaf, form action), the root-layout provider pattern (theme provider plus a focus-on-navigate leaf that resets focus after client route changes), and the file-organization contract all component skills follow. Invoke in Phase 5 immediately after scaffold, whenever placing a "use client" directive, deciding if a component is server or client, wiring providers into the root layout, choosing layout vs template, deciding where focus goes after a client-side route change, or when the question is "where does this component/state/fetch go" in a Next.js App Router project.