Scaffold design engineering playground

Next.js 16 + TypeScript + Tailwind v4 + Motion project showcasing
interface components built on semantic design tokens:

- Light/dark/system theming from one CSS variable set, with a
  pre-paint script so a saved theme never flashes
- Button, Switch and SegmentedControl with keyboard support, ARIA
  semantics and spring motion that respects reduced-motion
- Vitest + Testing Library tests, including a token-coverage test
- /tokens page documenting colour and motion tokens

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/lib/cn.ts

@@ -0,0 +1,5 @@
+import { clsx, type ClassValue } from "clsx";
+
+export function cn(...inputs: ClassValue[]) {
+  return clsx(inputs);
+}