Implement native Vinext route-aware type generation

[pacexy]

Summary

Vinext currently has an optional dev-time bridge that can run next typegen when Next.js is installed locally. That improves developer experience in the short term, but it should not be the long-term solution.

Vinext already scans the route tree internally via routing/app-router.ts and routing/pages-router.ts. We should use that data to generate the route-aware .d.ts files directly, without depending on Next.js.

Goal

Implement native Vinext type generation for helpers such as PageProps and LayoutProps, so route-aware typing no longer depends on next typegen or a local Next.js install.

Notes

• The current next typegen integration is intended as a temporary stopgap for developer experience.
• The native implementation should preserve Vinext's positioning as a Vite-based reimplementation rather than requiring the real Next.js package.

Related

• Stopgap PR: feat: add optional dev-time next typegen #651