Atom warm-start: bitonic sort rewards → quantile positions → write to
atom_positions_buf as initialization. Existing SGD optimizer refines.
Atoms start where reward mass actually is instead of uniform [-50,+50].
Robust PopArt: median/IQR normalization from sorted rewards replaces
Welford mean/var. More robust for bimodal distribution (many ±0.1
micro-rewards + few ±5.0 trade exits). Conditional: popart_robust=true.
Both reuse the same bitonic sort (~14ms per epoch, amortized).
gather_quantiles kernel extracts positions. extract_median_iqr reads
Q25/median/Q75 from sorted buffer.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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