docs: absorb prefetch stabilization and App Shell default in caching + runtime prefetching
Canary stabilized export const prefetch (#94571), renamed
'force-runtime' to 'allow-runtime' (#94568), and made App Shell the
default prefetch unit when partialPrefetching is on (#94510, #94448,
#93999).
caching.mdx: rename unstable_prefetch to prefetch and 'force-runtime'
to 'allow-runtime' in the Runtime prefetching subsection.
runtime-prefetching.mdx:
- Same renames throughout (description, example, table cell, related link).
- App Shells section rewritten around the new default: pair
partialPrefetching: true with experimental.appShells: true, and Link
prefetches the App Shell by default unless prefetch={true} is set.
Drop the old "loading state of last resort" framing and the
obsolete forward-looking GtK.
- "When to reach for" section tightens the cost framing: each visible
<Link> with 'allow-runtime' wakes a server, only pays off if the
prerender produces a better UI than the static shell.
- Drop the "Pairing with unstable_instant" section. With warning-level
validation as the default, the section's "validation only" vs
"validation + runtime prefetching" branching is stale; the guide
intro already assumes the route is structured for instant navigation.