OnChildFinishedAsync ignored CancelAsync's result. If a chain successor
sat in a non-cancellable state (e.g. parked to Idle out of band) when
its predecessor failed/was cancelled, CancelAsync was a silent no-op and
the cascade stopped there, leaving the rest of the chain Queued+blocked
forever. Now it walks past any link CancelAsync can't touch and keeps
cancelling downstream.