L40S 15-epoch repro #2 (train-multi-seed-kdkdv) revealed train_loss
exploded 1000-2000× in fold 1 (3.1 → 318k-694k) while val Sharpe
stayed healthy and CLIMBING (73 → 114). Train/val disconnect = pure
measurement bug, not real instability.
Mechanism: reset_for_fold() zeroed cached_iqr/cached_median at every
fold boundary. The `if cached_iqr > 0.0` guard at fused_training.rs:1220
then forced fold N+1's first epoch to the Welford GPU path. Welford
running stats inherited from fold N, combined with fold N+1's slightly
different reward distribution post-S&P + adversarial regime, produced
normalized rewards far outside C51 atom support [-50, 50] — categorical
loss readings 10^5x inflated. On rare timing-sensitive paths the
near-zero divide overflowed to Inf → NaN (the run-1 flagged=[2=on_b_logits,
3=mse_loss, 6=grad_buf, 7=save_current_lp, 8=save_projected]
diagnostic — what we caught was downstream of THIS root cause).
The diagnostic infrastructure from 756b1ef31 + 32e5375ac worked perfectly:
its precise signal of "on_v_logits clean, on_b_logits NaN, but loss is
also NaN, params clean pre-forward" surfaced the train/val disconnect
that pointed at the reward normalization bug.
Fix A (carry-forward, fused_training.rs:919-922):
Stop resetting cached_iqr / cached_median at fold boundary. Carry
fold N's final-epoch median/IQR forward as fold N+1's epoch-1
default. Same instrument, similar reward distribution between
adjacent walk-forward folds, so the carry is safe and gets replaced
by fresh stats at the end of fold N+1's epoch 1.
prev_popart_var still resets (sole consumer is tau-change detection;
a fresh fold counts as a change point regardless).
Fix B (permanent floor, dqn_utility_kernels.cu:1657):
Raise iqr fmaxf floor 1e-6 → 1e-4. With iqr=1e-6 a trade-exit
reward of 5.0 normalizes to 5e6 (vs post-fix 5e4) — much harder to
hit fp32 overflow. Defensive bound for genuinely-pathological iqr
paths (e.g. genuinely degenerate data quantiles), not a tuned
knob — Invariant 1 carve-out for numerical-stability bounds.
Per pearl_blend_formulas_must_have_permanent_floor.md (the same
recipe that resolved Kelly cap warmup + var_scale collapse before).
Resolves task #84 ("Fold-boundary state reset gap causes fold 1 grad
explosion").
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