Fix prefetch hints not being loaded during revalidation
The prefetch hints manifest was not loaded during revalidation renders,
causing all segments to fall back to PrefetchDisabled — effectively
disabling prefetching for that route until the next full build.
The app-page template handler constructs its own renderOpts from scratch
without including prefetch-hints.json. Normal requests go through the
server's renderOpts (which loads the manifest at startup), but background
revalidation renders go through the template handler, so the hints were
always empty.
Load the manifest in the route module's loadManifests() and pass it
through to renderOpts.
Also fix the fallback behavior when no hints exist in the manifest.
Without cacheComponents, dynamic pages have no static shell and
therefore no prerender pass to compute hints. Previously this fell back
to PrefetchDisabled, which blocked all prefetching. Now we pass null
hints to skip the inlining system and let prefetching proceed normally.
