* perf: add TTL-based eviction sweep for prefetch cache
Before this change, expired prefetch cache entries were only cleaned on
individual access or displaced by FIFO when at the 50-entry cap. On
link-heavy pages, expired entries wasted slots and caused premature
eviction of still-valid entries.
Now storePrefetchResponse() sweeps all expired entries before falling
back to FIFO eviction, so fresh entries survive longer under pressure.
* fix: sync prefetched URL set on cache eviction
Evicted entries were not removed from getPrefetchedUrls(), causing
Link's prefetched.has() check to return true for stale URLs and skip
re-prefetch. Clean up the prefetched set in both sweep and FIFO paths.
Also derive test counts from MAX_PREFETCH_CACHE_SIZE constant and
assert prefetched URL set consistency after eviction.
* fix: address review comments on prefetch cache TTL sweep
- Hoist Date.now() to top of storePrefetchResponse so sweep and new entry timestamp use the same instant
- Export MAX_PREFETCH_CACHE_SIZE in next-shims.d.ts to match runtime export
- Use fixed arbitrary time values in tests instead of real wall clock
- Clarify why prefetchedUrls count excludes the new entry in the below-capacity test
---------
Co-authored-by: James <james@eli.cx>
Divanshu Chauhan (divkix)