incomingFromCollectionCancelledBetweenTakeAndHandOverLosesNothing

(4i) cancels the reader before it resumes, so it cannot see a room that suspends between taking a frame and handing it over — the second window in which a frame can leave an inbox unseen. Here the reader runs on a StepDispatcher. The frame is routed on the test scheduler, which queues exactly one task for the parked reader; the reader runs exactly that task and is cancelled wherever it left off. Across that collection and the next, both frames must arrive exactly once, in order: delivered by the cancelled collection, or left for the next one — never lost.