Deploy to Vercel as a TanStack Start app via Nitro

Vercel served 404s: with Hydrogen gone it treated the app as a static
Vite site, but TanStack Start's build has no index.html; pages come from
its server. Add the hosting adapter TanStack documents for Vercel:

- nitro() in vite.config, for 'vite build' only (dev and the nodesandbox
  runtime never load it); on Vercel it writes .vercel/output with the
  static assets and one Node.js function running Start + Hono
- vercel.json: tanstack-start preset, Yarn 4 via Corepack
- 'yarn start' runs the Node build; canonical URLs use the Vercel domain

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "dev": "vite dev --host 0.0.0.0 --port 3000",
     "build": "vite build",
+    "start": "node .output/server/index.mjs",
     "typecheck": "tsc --noEmit"
   },
   "packageManager": "yarn@4.18.0+sha512.fcb8716fe7cd0eece141ffc18b92193a9df9204c1ba83189c288835223fc0bbe64af473bab0d5e9927a7daeb5caf2bb07eb2787cc9338ca040ea125f2a1f2f7e",
@@ -24,6 +25,7 @@
     "framer-motion": "^12.42.2",
     "hono": "^4.13.12",
     "lucide-react": "^0.562.0",
+    "nitro": "3.0.260903-beta",
     "radix-ui": "^1.6.7",
     "react": "^19.3.0",
     "react-dom": "^19.3.0",