Function subgraph_test_node 

pub fn subgraph_test_node<State>(
    child: CompiledGraph<State, State>,
) -> Box<dyn Fn(State, NodeContext) -> NodeFuture<State> + Send + Sync>
where
    State: Clone + Send + Sync + 'static,

Embeds child as a shared-state subgraph node (a thin wrapper over shared_subgraph_node).

The child runs over the parent state and its final state becomes the parent update, recording a ChildRun onto the enclosing run so the subgraph is visible on the parent GraphExecution::child_runs.