NoLeaveSignal

The source genuinely has no leave signal — a fixed-roster fake, a platform stub, a browse API that only ever reports arrivals — and its departures() is therefore emptyFlow().

This arm is a declaration, not an exemption. It is what DiscoverySourceConformanceSuite.anArrivalIsNeverReportedAsADeparture holds the source to, and a source that declares it while emitting anything at all fails there.