When RTVI is enabled, PipelineWorker now republishes inbound ui-event /
ui-snapshot / ui-cancel-task messages onto the bus as a broadcast
BusUIEventMessage, and translates outbound BusUICommandMessage / BusUITask*
carriers into the matching RTVI frames. This lets a UIWorker on the bus observe
and drive the client UI with no decorator or manual wiring; when no UIWorker is
present the events are simply unconsumed.
The BusUI* carriers live in the bus layer so both pipeline and workers can
reference them without an import cycle.
