Extends phase1d_long_horizon with a 50/50 val split (cal/test halves):
fit Platt and Isotonic on cal, evaluate on held-out test. Reports both
uncalibrated AND calibrated metrics (accuracy, AUC, Brier, log-loss).
Hypothesis: the K=6000 Mamba result (AUC=0.66 / acc=0.62 from the
uncalibrated 4cf9499b5 smoke) has a 4-point AUC-accuracy gap which
mirrors the Phase 1c pattern; Platt should compress that gap and lift
accuracy further without retraining.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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