_TasksLifecycleBaseDiscover subgraph invocations as in-process navigation handles.
Per discovered direct-child subgraph, builds a SubgraphRunStream
(or AsyncSubgraphRunStream) wrapping a child mini-mux scoped to
the subgraph's namespace. Consumers iterate run.subgraphs to
receive handles, then drill into handle.values / handle.messages
/ handle.subgraphs (recursive grandchildren) / handle.lifecycle.
Each mini-mux owns its own scope and uses its own
SubgraphTransformer to discover its direct children, so
grandchildren live on the child handle — never on the root's
subgraphs log. Forwarding events into the matching child mini-mux
is what keeps the child's projections populated.
Native transformer — subgraphs is exposed as run.subgraphs.