Keep never-prerenderable params out of self-hosted shells - #98612

With Cache Components enabled and Partial Prefetching disabled, blocking
prerenders in `next start` could cache params that no
`generateStaticParams` result supplies. For `/[top]/[bottom]`, where
only `top` is supplied, this included `bottom` in the static HTML and
created a separate cache entry for each bottom value.

Adapter deployments already enforce this rule. The legacy Vercel builder
has a known limitation that the existing prerender tests explicitly
exclude.

 #95872 established that these params must remain dynamic. #96297 later
put self-hosted eligibility-based shell keys and fallback-param
selection behind the `partialPrefetching` flag, alongside automatic
fallback upgrades. #98512 then made resumes honor the parameter set
recorded by the selected prerender. That did not create the invalid
prerenders, but allowed their over-resolved params to enter Cached
Navigations.

The RSC test added in #98512 did not account for that known
legacy-builder limitation and consequently failed in this [canary deploy
run](https://github.com/vercel/next.js/actions/runs/34659100427/job/103469344769).

For `next start`, shell keys and unresolved params now follow GSP
eligibility even when upgrades are disabled. Cache writes, revalidation,
and navigation RDC reads use the same key, including shared shells that
cannot be completed further. Routes whose params are all prerenderable
retain their existing keys when Partial Prefetching is disabled.
Automatic upgrade triggers remain gated.

The aforementioned `cached-navigations` tests now separate cold RSC
navigation from initial HTML and assert eligibility independently of the
observed response. Added coverage checks shell sharing, explicit
revalidation, and generated optional paths. Legacy-builder failures are
documented with expected-failure gates; its implementation remains
unchanged.