routing

Designs the app/ route tree for a Next.js 16 App Router site — route groups for layout scoping, dynamic segments with awaited params (they are Promises in Next 16), parallel routes with their mandatory default.tsx, intercepting routes for URL-addressable modals, per-segment loading.tsx/error.tsx/not-found.tsx plus global-not-found.tsx and global-error.tsx as designed moments, and a root-layout Speculation Rules block that prerenders the likely next document — all designed to the system, not framework defaults. Invoke during the build phase when translating design/SITEMAP.md into app/ directories, when adding a dynamic or catch-all route, when a modal needs a shareable URL, when a 404/error/loading surface still looks default, or when navigation should feel instant. Trigger phrases — "set up the routes", "dynamic route", "route group", "slug page", "modal with a URL", "intercepting route", "custom 404", "500 error page", "loading state for this page", "error page", "prerender the next page".