Capture debug events as a drainable stream.
Surfaces stream_mode="debug" data on run.debug as a
StreamChannel[dict[str, Any]]. Each item is a debug event with
step-level detail (checkpoint snapshots, task payloads, and
task results wrapped with step number and timestamp).
Only events at the run's own scope are captured; debug data from
deeper subgraphs is available on the respective subgraph handle's
.debug projection.
Native transformer — run.debug is a direct attribute.
