Slow-path CPU scalar AUC over (probs, labels) vectors. Mann-Whitney U
formulation with average-rank tie handling. NaN labels filtered
(mirrors generate_labels masking semantics).
Tests (6/6 lib unit):
- perfect separation -> 1.0
- perfect anti-separation -> 0.0
- random uniform pairs -> ~0.5 (within 0.05)
- NaN labels filtered out before computation
- empty class -> 0.5 (defensive fallback)
- all-tied scores -> 0.5 (average-rank correctness)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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