Serve cached misses from the filesystem route cache (#96230)
`getItem` stores `null` for paths that don't resolve, but checks the
cached value with `if (lruResult)`, so a cached `null` is never served:
every request for an unknown path — including every dynamic-route
request — re-runs the lookup. It's been this way since the cache was
introduced in #52492.
Return the cached entry whenever the key is present. This can't go
stale: in production the item sets are only populated during
`setupFsCheck`, and the LRU is disabled in dev.
Cached misses are stored as a dedicated sentinel instead of `null`.
Since serving a cached miss has no observable effect, no test can catch
this regressing — that is also why it went unnoticed. With the sentinel
it's a type error instead: reintroducing the truthy check fails `pnpm
types` (verified). Same pattern as `invalidSourceMap` in
`source-maps.ts`.
Stacked on #96229, which bounds what negative entries can retain.
## Test Plan
No direct test: the change is unobservable by design, and the existing
e2e suites assert that it stays that way. The regression guard is the
type system, as above. `test/e2e/app-dir/not-found/basic` passes in prod
mode (14/14), exercising the negative-cache path.
