TaskStarted

A delegated task's child run was admitted (design 4.47): live, sent for the parent's task call as the child's run claims its thread - only for a run that was admitted - and again on each reattach. It is the nested stream's handle: a parent subscription made with nested = true receives the child's events after it, as StreamEvent.Nested(child, _). child.runId is the run the call is about to drive, or, for a child already settled, the run that settled it.