IterEval

What ReplHost.execute hands back for a cross-tick snippet: the iterator to drive one step per tick. Both output channels — println, and the => type = value each yield renders — land in the per-eval out sink EvalTask owns and passed down, so the two arrive interleaved in the order they happened.

errorFlag is set by the masking-side iterator guard if a step throws — surfaced as errored, so the lane can return the partial output and still mark the eval's Outcome as an error.