Stop emitting a redundant route per prefetch segment (#97720)
For a dynamic app page that has a fallback shell, say `/[lang]/[slug]`
with a shell for the root param value `de`, the build emits three routes
to an adapter. Simplified, with the request pattern on the left and the
artifact it resolves to on the right:
```diff
- /de/<slug>.segments/$d$lang/$d$slug<seg> -> /de/[slug].segments/$d$lang/$d$slug<seg>
/de/<slug><.rsc|.segments/*.segment.rsc> -> /de/[slug]<matched suffix>
/de/<slug> -> /de/[slug]
```
This change removes the first one. The second already covers it: its
suffix group accepts `.segments/<path>.segment.rsc` as well as `.rsc`,
and it copies the matched suffix into the destination, so a per-segment
request resolves to the same artifact either way.
That second route is also the only one that ever answered `_tree` and
`_full` requests, because a per-segment route pins one literal segment
path in its regex. A `_tree` prefetch for `/de/<slug>` resolves to
`/de/[slug].segments/_tree.segment.rsc`, which the per-segment route
cannot produce. So this removes a duplicate, not a mechanism.
Only fallback shells emit per-segment routes, because their artifacts
sit under an unresolved param and need a rewrite to reach. Segment
artifacts for a concrete prerendered path need none, since a request for
that path matches them directly. Apps with many shells therefore lose
close to a third of their routes. We measured an in-progress feature
branch of the v0 chat app, which enumerates precomputed flag, locale and
device permutations in `generateStaticParams` for a top-level dynamic
segment and so multiplies every route below it. Building that branch
before and after the change removes 32% of its routes, adds none, and
changes nothing else once the build ID is normalized.
This also adds `experimental.collapseAdapterRoutes`. It defaults to
`true`, and it controls this collapse together with the ones that follow
in this stack. A build that sets it to `false` emits the same route
table as a build without any change in this stack.
`prefetchSegmentDataRoutes` stays in `routes-manifest.json`. A build
that does not use the adapter reads the field from that manifest and
derives the same routes from it. This change therefore leaves that path
alone.
**Verified upstack with a [full deploy test
run](https://github.com/vercel/next.js/actions/runs/32540912007).**
