Fix startup warmup for empty-shell app route cache (#91470)
Startup loading of APP_PAGE PPR entries seeded the in-memory cache from
disk using a size estimate that only counted html and the route-level
RSC payload. Empty-shell prerenders have 0-byte html and do not persist
a monolithic .rsc file, with the reusable data instead stored in
postponed metadata and per-segment .segment.rsc files. That made the
computed size 0, caused LRUCache to reject the seeded entry, and forced
the first request after next start to rerun prerender and rewrite the
build artifacts.
Account for postponed state and segment buffers when sizing APP_PAGE
cache entries so empty-shell prerenders can warm the startup cache the
same way contentful shells do. Add a production regression that asserts
both routes are emitted as build artifacts and that next start does not
rewrite them on the first request.
The regression skips deployment mode because it verifies self-hosted
behavior by inspecting local .next artifact mtimes, which is not a
portable signal in deployment mode. When deployed through an adapter it
is not expected that the local Next.js process is handling entries
through the Response cache reading from the `.next` folder anyway
