fix(iqn): hard-sync target from online at fold boundary — kills geometric Q-drift in fold 1

Add `GpuIqnHead::sync_target_from_online` (DtoD copy of online_params →
target_params) and call it from `FusedTrainingCtx::reset_for_fold`
alongside the existing IQN Adam reset.

Root cause: at fold boundary the main DQN does shrink-and-perturb +
hard target sync (gpu_dqn_trainer.rs:12495 — the latter explicitly added
to prevent fold-1 grad explosion). The IQN head was added later but
only had Polyak EMA `target_ema_update` — no hard-sync existed. Polyak
at τ=0.005/step closes ~0.5%/step, far too slow to close the
fold-boundary online↔target gap before inflated Bellman TD-error
compounds geometrically through the IQN backward pass.

Bug signature this resolves:
  F0 ep5 Q=+0.79  (healthy)
  F1 ep1 Q=+0.82  (boundary fine; c51_alpha warmup blends IQN low)
  F1 ep2 Q=+2.23  (drift starts — c51_alpha ramp + IQN online↔target gap widens)
  F1 ep3 Q=+4.05
  F1 ep4 Q=+10.66 (geometric ~2.3×/epoch)
  F1 ep5 NaN at step 5 (fp32 overflow past atom support)

Per `feedback_no_partial_refactor.md`: when a shared contract
(fold-boundary state migration) changes, every consumer must migrate
together. The main DQN sync was added explicitly — the IQN, TLOB, IQL,
and aux Adam states were added later but never extended to that
contract. This commit closes the IQN gap (primary). Aux Adam + IQN
readiness gaps follow in subsequent commits.

Touched:
- gpu_iqn_head.rs: new `sync_target_from_online` (mirrors gpu_dqn_trainer.rs:12495 exactly)
- fused_training.rs: call the new sync inside the existing IQN reset block
- docs/dqn-wire-up-audit.md: Invariant 7 entry covering Fix 1 of 3

cargo check -p ml --lib clean at 13 warnings (workspace baseline). No
fingerprint change. DtoD memcpy only — no HtoD/HtoH/DtoH paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/dqn-wire-up-audit.md

@@ -2,6 +2,39 @@
 
 **Status:** Populated during Plan 1 Task 6 (A.5 orphan audit). Updated on every commit per Invariant 7.
 
+Fold-boundary reset gap — IQN target hard-sync (2026-04-28, Fix 1 of 3):
+the main DQN's `sync_target_from_online` (DtoD `online_params →
+target_params` at `gpu_dqn_trainer.rs:12495`) is wired into
+`FusedTrainingCtx::reset_for_fold` (`fused_training.rs:887`) explicitly
+to prevent fold-1 grad explosion. The IQN head, added later, only
+exposed `target_ema_update` (Polyak τ=0.005/step) — no hard-sync. After
+fold boundary the IQN online side advances through fold N+1's first
+gradient steps while the IQN target buffer still holds end-of-prior-
+fold weights, so the Bellman TD error gap widens by a factor that
+Polyak takes hundreds of steps to close. Empirically that drives a
+geometric ~2.3×/epoch Q-drift inside fold 1
+(`F0 ep5 Q=+0.79 → F1 ep1 +0.82 → ep2 +2.23 → ep3 +4.05 → ep4 +10.66
+→ ep5 NaN @ step 5` on fp32 overflow past atom support). The boundary
+handoff at F1 ep1 looks fine because the c51_alpha warmup ramp blends
+IQN low; once c51_alpha ramps up the IQN online↔target gap dominates.
+**Fix**: add `GpuIqnHead::sync_target_from_online` (DtoD copy mirroring
+the main DQN's helper exactly — same `memcpy_dtod_async` pattern, same
+stream, same total_params) and invoke it from
+`FusedTrainingCtx::reset_for_fold` immediately before the existing IQN
+Adam reset. Per `feedback_no_partial_refactor.md`: the
+fold-boundary state-migration contract has multiple consumers (main
+DQN trunk, IQN head, aux Adam states, IQN readiness EMAs) and
+extending it must migrate every consumer in lockstep. The IQN target
+sync is the primary root cause; the aux Adam state resets and IQN
+readiness reset land in the next two commits.
+
+Touched: `gpu_iqn_head.rs` (new `sync_target_from_online` method
+between `reset_adam_state` and `target_ema_update`), `fused_training.rs`
+(invoke the new sync inside the existing `if let Some(iqn) = …` block
+in `reset_for_fold`, immediately before `iqn.reset_adam_state()`).
+cargo check -p ml --lib clean at 13 warnings (workspace baseline). No
+fingerprint change. DtoD only — no HtoD/HtoH/DtoH paths introduced.
+
 MSE warmup loss target clamp to C51 atom support (2026-04-28): the
 fold-boundary PopArt carry-forward + iqr floor (`d710f9d50`) reduced
 fold-1 epoch-1 train loss readings 16× (318k → 19k) but ep-2/ep-3