[backport] Fix catch-all index page being served for every other slug - #97416

Navigating between sibling pages of an optional catch-all could leave
the URL updated but the content stuck on the route's index page, with no
request made at all. On nextjs.org that is every /docs/* sidebar link,
once /docs itself has been prefetched.

The App Shell of a segment is the part that doesn't depend on params, so
the cache keeps one copy and reuses it for every param value. The client
decides it has that shell when a response arrives without a shell/full
split.

But a response arrives without a split whenever the render had no
dynamic hole, and that is also what a page fully prerendered at concrete
params looks like. The split alone can't tell the two apart, and the
second is not param-independent at all — its content is the params. So
the /docs page was stored as the shell for every slug. Because a payload
that needs nothing further is also recorded complete, sibling slugs read
it and stopped there: no prefetch, and no dynamic request during
navigation to correct it.

Treat a payload as param-independent only on evidence. The server
already reports which params each segment depends on, so use that.
Without it, assume every param varies, except for a shell variant, which
reduces param-dependent content to param fallbacks and so is good for
any value of them.

The new e2e suite prefetches a fully static catch-all index, navigates
there, then prefetches and navigates to a sibling slug. It fails on
canary with a prefetch that never fires, and is verified with vary
params both enabled and disabled.

(cherry picked from commit 5f735c1)