Capture custom events as a drainable stream of arbitrary payloads.
Nodes emit custom data via get_stream_writer(). This transformer
surfaces those events on run.custom as a StreamChannel[Any],
preserving payloads in arrival order.
Only events at the run's own scope are captured; custom data from
deeper subgraphs is available on the respective subgraph handle's
.custom projection.
Native transformer — run.custom is a direct attribute.
