Replace the single-scroll showcase and its anchor/scrollIntoView
navigation with real routes: /, /patterns, /components/*, /foundations/*,
/docs/*, plus a 404 fallback.
- src/app/routes.tsx is the route table and the only source for paths,
section headings, sidebar entries, breadcrumbs and the rail, so
navigation cannot drift from the pages that exist
- src/pages/* holds one component per route; App.tsx is now the shell
(topbar, sidebar, breadcrumbs, Switch, right rail, scroll-to-top)
- the active pattern moves into the URL (/patterns?pattern=table) and the
sidebar/palette deep-link into it; SearchPalette results now carry a path
- Sidebar/RightRail render <Link> with aria-current; Hero and Topbar use
route links too (ButtonLink keeps the button visual)
- PATTERNS + PatternId move to src/data/patterns.ts (no non-component
export in a component file), pattern buttons gain aria-pressed
- tests/routing.test.tsx: registry/nav invariants, per-route rendering,
sidebar navigation, pattern deep links, 404
