Station Animation Emitter
A StationObserverIfc that emits AnimationEvent.StationEntered and AnimationEvent.StationExited as QObjects enter and leave a station. Attach with station.attachStationObserver(...); this is the per-station flow animation enabled by the additive station observer (decision D10/Phase 3B), complementing the network-boundary events.
Stateless and shareable: the station is passed to each callback, so one instance can observe many stations.