0061 — The prefetcher blocks on WaitForRoom; the buffer never drops a claimed row

Context. The consumer pipeline has one Prefetch goroutine batch-claiming rows into a bounded PressureQueue, and a Dispatch loop draining it into one goroutine per message gated by WorkerPoolLimiter. A bounded buffer needs a policy for what happens when it fills.

Decision. Prefetch gates on WaitForRoom before claiming, replacing the planned CanEnQueue check. Backpressure flows backwards: a full buffer stops claiming, so the EnQueue drop path is never reached.

Consequences. Every buffered row is already claimed — its lease_until is stamped and attempts incremented — so dropping one would strand a leased row until reclaim. Gating before the claim makes that impossible by construction. The claim size becomes min(batchLimit, freeSlots), tying supply directly to available worker capacity. Rejected: check-then-drop via CanEnQueue/EnQueue — you cannot drop a row you have already leased.