Smoke smoke-test-vh9bj revealed: after F+H Q-drift kill fix, fold 0
trains successfully (5 epochs, Best Sharpe 36.03), but fold 1 fails
at epoch 1 with "Gradient collapse detected for 5 consecutive epochs"
despite epoch 1 grad_norm=296,417 (healthy). Root cause: the per-step
gradient_collapse_counter on DQN was already near patience (5) from
fold 0's late-epoch near-zero-grad steps, and DQNTrainer::reset_for_fold
didn't clear it. Plus, training_steps accumulating across folds made
the `past_warmup` gate always-true in fold 1+, removing the warmup
grace period for data distribution shifts.
Add DQN::reset_for_fold zeroing both. Wire from DQNTrainer::reset_for_fold
alongside the A.1 prev_epoch_q_mean reset. Pure additive — same
fold-boundary state-reset gap pattern as A.1, A.2, F.
Predicted impact: fold 1+ now starts with counter=0 and warmup window
restored; gradient collapse check has its full per-fold grace period.
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