Manifest endpoint no longer returns prerendered HTML

Patch Changes

• 2d918b8: Fix React Router /__manifest returning prerendered HTML when the root route is statically generated.

  When prerender() emitted static HTML for the index route, the SSR function was removed from the catch-all target, so runtime-only paths like /__manifest fell through to the prerendered index.html. The builder now keeps the index SSR function for the catch-all, adds an explicit / → /index.html prerender rewrite, and skips overwriting prerendered .data artifacts.