### What?
Navigating to a layout that is part of a route group that uses route
interception currently will trigger a 404 error if the route group
doesn't define a `default` segment.
### Why?
When `next-app-loader` injects fallback defaults into the loader tree,
it does so by first seeing if a default already exists. However it does
this without ignoring route groups, meaning if you have a
`/app/default.tsx` and your interception route is at
`/app/(level1)/(level2)`, it will look for the default at
`/app/(level1)/(level2)/default.tsx`.
When a `default` isn't found, the fallback behavior is to trigger a
`notFound()` error. This means navigating to the intercepting route that
has no `default` for the `children` segment will 404.
### How?
This adjusts the fallback behavior by attempting to find the `default`
by normalizing the segment path, which will ignore route groups. That
way `/app/(level1)/(level2)/default` will first check `/app/default.tsx`
before falling back to `notFound` behavior.
Fixes#59279
Closes NEXT-1813
490d23805f