The area under the precision-recall curve, as the step sum over the distinct predictions: at
each, the precision of calling every unit at or above it a presence, weighted by the share of
presences that cut adds. Units sharing a prediction enter together, so the order they arrived in
does not move the score. Its floor is the prevalence rather than one half, which makes it the
reading of how well presences are ranked above absences where presences are rare, and it is
reported beside roc_auc() for that reason.