The roll-up treated a cancelled child the same as a failed one, so a DAG
the operator cancelled before it started reported `Failed` — it claimed to
have failed at something when nothing under it ever ran.
`Failed` still outranks `Cancelled`: a group where one step broke and the
rest were dropped in response is a failure, and that is the fact worth
surfacing. Only a group with no failed child at all reports the cancel.
This also settles a disagreement. `DagView::rollup_state` on the wire has
always ranked failed over cancelled over the rest; the graph's own roll-up
had no `Cancelled` outcome to rank, so the two described the same DAG
differently depending on which one you asked.
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