docs(site): Fumadocs documentation site on TanStack Start

New standalone app at docs-site/ — Fumadocs (fumadocs-core/ui 16, fumadocs-mdx
15) on TanStack Start (Vite 7 + nitro-v2 bun preset, React 19, Tailwind 4),
mirroring the web/ console stack but with no auth/i18n/orval — docs stay public.

- catch-all docs route (routes/docs/$.tsx), Orama search (routes/api/search.ts),
  RootProvider shell, MDX component map, shared nav, custom 404
- content/docs/: hand-written index.mdx + meta.json nav, plus 7 pages imported
  from repo docs/ + README (leading H1 stripped, YAML frontmatter added; kept as
  .md so existing </{ don't trip MDX JSX). Content is a one-time snapshot.
- mdx() is plugins[0]; tsconfig collections/* -> ./.source/*; SSR search variant;
  @source for fumadocs-ui classes. Generated .source/routeTree/dist/.output ignored.

Verified: bun run build (client+SSR+nitro) green, tsc clean, dev + prod servers
serve all routes 200 with SSR content + nav, search returns hits, 404 works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/src/router.tsx

@@ -0,0 +1,29 @@
+import { createRouter as createTanStackRouter, Link } from '@tanstack/react-router'
+import { routeTree } from './routeTree.gen'
+
+export function getRouter() {
+  return createTanStackRouter({
+    routeTree,
+    defaultPreload: 'intent',
+    scrollRestoration: true,
+    defaultNotFoundComponent: NotFound,
+  })
+}
+
+function NotFound() {
+  return (
+    <main className="flex flex-1 flex-col items-center justify-center gap-4 px-4 py-24 text-center">
+      <h1 className="text-3xl font-bold">404</h1>
+      <p className="text-fd-muted-foreground">This page could not be found.</p>
+      <Link to="/" className="text-fd-primary underline underline-offset-4">
+        Back home
+      </Link>
+    </main>
+  )
+}
+
+declare module '@tanstack/react-router' {
+  interface Register {
+    router: ReturnType<typeof getRouter>
+  }
+}