Native empirical ROC curve, computed exactly as pROC::roc does
with levels = c(0, 1), direction = "<": candidate
thresholds are -Inf, the midpoints between consecutive distinct
scores and +Inf (with pROC's near-tie correction), a case is
called positive when its score is >= the threshold, and the AUC
is the trapezoid area under (specificity, sensitivity), which equals
the Mann-Whitney U probability with ties credited 0.5. Observations
with a missing label or score are dropped from the curve (pROC's
na.rm = TRUE).
