NodeCompleted
data class NodeCompleted<out State>(val step: Int, val node: String, val state: State) : GraphEvent<State>
node finished in step. state is what the node returned, or for a node with a work and an update, its update applied to the state the node received. In a step with several nodes this is the node's own result, before it is combined with the others; the combined state arrives with StepCompleted.