v_min/v_max ±15 with gamma=0.99 only covers 15% of theoretical Q range
(Q_max = 1/(1-0.99) = 100 for unit-variance PopArt rewards). C51 top atom
saturates on sustained winners, degrading distributional learning. ±50
covers 95th percentile.
PopArt GPU buffers (popart_mean, popart_var, popart_count) were never
zeroed between walk-forward folds — fold 2's reward normalization was
contaminated by fold 1's statistics. Now reset alongside Adam state in
reset_adam_state().
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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