Discovers child subgraph handles and fans out events to active ones.
Mirrors the per-event body of _SubgraphsProjection._subgraphs_iter
(_async/stream.py:963-1041) plus _apply_tasks_result. Root-inbox
forwarding and terminal-status-on-close stay at the projection / wrapper
layer.
Tuple-form namespace of this decoder's parent. () for root.
Keyword-only (path, graph_name, trigger_call_id) -> handle.
