fix(popart): carry fold stats forward + raise iqr floor — kills fold-1 loss explosion

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").

docs/dqn-wire-up-audit.md

@@ -2,6 +2,50 @@
 
 **Status:** Populated during Plan 1 Task 6 (A.5 orphan audit). Updated on every commit per Invariant 7.
 
+Fold-boundary PopArt carry-forward + iqr permanent floor (2026-04-28):
+the L40S 15-epoch repro #2 (`train-multi-seed-kdkdv`, commit
+`32e5375ac`) showed a **massive train/val disconnect** in fold 1:
+mean_loss climbed from 3.1 (fold 0 healthy) to 318,898 → 590,136 →
+694,323 across fold-1 epochs while val Sharpe stayed healthy and
+*climbing* 73 → 114. The 10⁵× loss readings turned out to be a
+fold-boundary PopArt reset bug, not a real instability. Mechanism
+traced via the diagnostic data (the same diagnostic infrastructure
+landed in `756b1ef31` + `32e5375ac` worked perfectly to surface the
+real signal): `reset_for_fold` zeroed `cached_iqr` / `cached_median`
+at every fold boundary, forcing fold N+1's first epoch to fall
+through the `cached_iqr > 0.0` guard at fused_training.rs:1220 to
+the Welford GPU path; combined with the Welford running stats
+inherited from fold N — and fold N+1's slightly different reward
+distribution post-S&P + adversarial — the normalised rewards landed
+far outside the C51 atom support [-50, 50], producing astronomical
+categorical loss readings. The val path doesn't use PopArt so val
+Sharpe was unaffected. On rare timing-sensitive paths the
+near-zero-divide overflowed to Inf → NaN (the run-1 `flagged=[2,3,
+6,7,8]` diagnostic firing). **Fix A**: stop resetting
+`cached_iqr` / `cached_median` at fold boundary in
+`FusedTrainingCtx::reset_for_fold` (fused_training.rs:919) — 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 (its sole consumer is tau-change detection — a fresh
+fold counts as a "change point" anyway). **Fix B**: raise the iqr
+permanent floor in `popart_normalize_robust`
+(`dqn_utility_kernels.cu:1657`) from 1e-6 → 1e-4. The previous
+1e-6 was insufficient defence: a trade-exit reward of 5.0 with
+iqr=1e-6 normalises to 5e6, vs the post-fix 5e4 (still very large
+but much harder to hit fp32 overflow). Per
+`pearl_blend_formulas_must_have_permanent_floor.md` and
+`feedback_isv_for_adaptive_bounds.md` (numerical-stability bound
+carve-out under Invariant 1). Touched files: `fused_training.rs`
+(remove 2 of 3 lines from the `reset_for_fold` PopArt block, expand
+docstring); `dqn_utility_kernels.cu` (raise the iqr fmaxf floor,
+expand kernel comment). cargo check clean at 13 warnings (workspace
+baseline). No fingerprint change. Resolves task #84 ("Fold-boundary
+state reset gap causes fold 1 grad explosion"). Diagnostic kernels
+from `756b1ef31` + `32e5375ac` remain in place to catch any future
+NaN paths through the loss-component buffers.
+
 NaN diagnostic flag 12 = `save_h_s2` (2026-04-28): the previous wire-up
 commit (`756b1ef31`) extended NaN coverage to flags 0-11 and proved that
 the L40S fold-1 NaN entered via `flagged=[2=on_b_logits, 3=mse_loss_scalar,