seo: add app/sitemap.ts enumerating routes + 225 component pages

[bntvllnt]

Problem

No sitemap.xml. Crawlers must discover 225 component pages by following links — slower indexing, missing priority/lastmod hints.

Goal

Programmatic, build-time-generated sitemap covering every route.

Acceptance criteria

• apps/registry/app/sitemap.ts exists, exports default MetadataRoute.Sitemap.
• Includes static routes: /, /components, /docs, /philosophy.
• Iterates registry.json items → emits one entry per /components/[slug].
• Includes raw registry endpoints: /r/registry.json and per-item /r/[name].json (low priority).
• Each entry has lastModified, changeFrequency, priority calibrated (home + /components = 1.0; component pages = 0.7; docs = 0.8; raw JSON = 0.3).
• curl https://ui.vllnt.ai/sitemap.xml returns valid XML with ~230 URLs.

References

• Next.js: https://nextjs.org/docs/app/api-reference/file-conventions/metadata/sitemap

Depends on

• #robots-issue (allow crawlers first)