Report a background refresh behind stale data you are returning now.
The engine flips router.isRevalidating and patches snapshot.data when
the promise resolves — dropping it if the location has since changed, and
leaving the stale data in place if it rejects. Without this a
stale-while-revalidate response would refresh the HTTP cache but never the
page.
Per-record channel handed to a loader handler, bound to the record and location of this run — so it cannot be misattributed when a navigation and a query refetch overlap. Optional, and appended last: handlers written against the four-argument signature keep working untouched.