incomingFromCollectionCancelledAsAFrameArrivesLeavesItForTheNext

The first collection is cancelled at the instant a frame reaches the member's inbox, and the next collection still receives that frame.

The rig: a probe on Room.incoming cancels the reader when it sees the frame. The reference room routes a frame onto incoming before offering it to the member's inbox, and the test scheduler runs resumed coroutines in order, so the probe's cancel lands after the inbox was offered the frame and before the reader runs — the window in which a Channel-backed inbox lost it. The rig's own preconditions are asserted, so a harness that orders delivery differently fails here loudly rather than passing without having reached the window.