The expected_q kernel computed mean_adv as a SINGLE scalar averaged
across ALL atoms AND actions, then subtracted from every logit.
This destroyed per-atom advantage structure, making all actions
produce nearly identical expected Q-values.
Fixed to compute per-atom mean: mean_a(A[a,j]) separately for each
atom j, matching the C51/MSE loss kernels' correct implementation.
This bug affected: backtest evaluation action selection, Q-gap
conviction filter (always 0 → floor 0.25), and Q-value monitoring.
Training loss kernels were NOT affected (they had correct per-atom mean).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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