Platt scaling drops held-out Brier from 0.346 → 0.221 (chance=0.250);
log-loss 1.096 → 0.632. Both Platt and isotonic land below chance baseline.
AUC-accuracy gap was pure miscalibration, not fundamental misexpression.
Learned: Platt a=0.32 (raw logits too extreme), b=0.89 (positive offset
needed). Trained MLP underconfidence on positives compounds with negative
prior. Proceed to 1d.1.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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