anArrivalIsNeverReportedAsADeparture

A peer arriving must not surface on departures().

On the DepartureFixture.Emits arm this catches a source that pipes its browse callbacks into the wrong feed. On the DepartureFixture.NoLeaveSignal arm it is the whole of that arm's obligation: a source declaring it has no leave signal must emit nothing, including while something is happening.

The negative window is awaitQuiescence — by default awaitBudget of virtual time, which is cheap and deterministic. A harness that has no virtual clock to burn must override that hook with a real one; it cannot end up with a zero-length window by omission.