### What & Why?
Interception routes depend on contextual information that are provided
via request headers. Specifically it needs to know about the
`Next-Router-State-Tree` when generating the interception route RSC
data, which isn't available at build time. This doesn't currently cause
any usage issues, but it erroneously emits static files & RSC payloads
that the client router won't be able to use and will instead fallback to
a dynamic request.
I removed some special case in an existing test since this fix also
resolves a discrepancy in behavior when PPR is turned on
### How?
This excludes interception routes from `appStaticPaths` at builds which
currently determines which pages should be statically generated.
Closes NEXT-2190
### 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
