Capture updates events as a drainable stream of node outputs.
Surfaces stream_mode="updates" data on run.updates as a
StreamChannel[dict[str, Any]]. Each item is a dict mapping a node
(or task) name to the update it returned after a step.
Only events at the run's own scope are captured; updates from deeper
subgraphs are available on the respective subgraph handle's
.updates projection.
Native transformer — run.updates is a direct attribute.
