Class DemandResidencyPump

Admission + prefetch orchestration in front of a DemandGeometryQueue.

Consumers request() products as demand changes (the pump owns admission — don't request on the queue directly); each async pump() cycle:

1. selects the top-priority pending batch,
2. awaits source residency for the batch (failures re-queue for retry and are reported, never thrown — one bad range must not stall the viewport),
3. forwards the resident products to the queue and runs its synchronous pump under the same batch cap.

Re-entrant calls coalesce: a pump() while one is in flight returns the in-flight cycle's promise rather than interleaving prefetch batches. Products already resident in the queue skip prefetch entirely (their ranking refresh is forwarded immediately).