feat: expose route type generation without Vite

[everton-dgn]

The route declaration is written through Vite's plugin lifecycle, so a tsc step that runs first can use a stale file-routes.d.ts.

filesystem-routing/vite now exports writeRouteTypes and WriteRouteTypesOptions. The function takes a route directory or a router plus the plugin's serialization options. It writes only when contents change and returns true when it wrote, or false when they already match.

The plugin calls writeRouteTypes with the same router and options as before. The existing serializer is unchanged, and the client router still takes precedence in per-environment configurations.

pnpm test passes (125 tests), and pnpm exec tsc --noEmit passes.

Closes #23