Task 3: Enable PopArt reward normalization (default true, all 3 TOML configs,
wiring confirmed in fused_training.rs submit_forward_ops_main()).
Task 4: Couple PopArt variance with tau reset — add prev_popart_var to
FusedTrainingCtx, read_popart_var() to GpuDqnTrainer, read_popart_variance()
+ should_reset_tau() to FusedTrainingCtx, tau reset injected at epoch boundary
in training_loop.rs after log_phase_timing().
Task 5: Add best_sharpe/best_epoch/best_val_loss reset to reset_for_fold() so
each walk-forward fold competes independently. Also wire v8 reward fields
(popart_enabled, micro_reward_scale, td_lambda, max_trace_length,
hindsight_fraction, hindsight_lookahead) through training_profile.rs apply_to().
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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