Wires the eval_dist HEALTH_DIAG group with per-magnitude action distribution
read from the validation backtest. H10 detection signal — training-mode
entropy may look uniform while eval-mode argmax collapses to Quarter.
Implementation:
* GpuBacktestEvaluator::read_eval_action_distribution_per_magnitude()
reads actions_history_buf to host, decodes magnitude bin per sample
(action layout: dir*27 + mag*9 + ord*3 + urg → mag = (a/9) % 3),
returns [Quarter, Half, Full] normalized over non-skipped entries.
* DQNTrainer.last_eval_magnitude_dist field, populated in metrics.rs
after each validation backtest. Non-fatal on readback error.
* HEALTH_DIAG eval_dist group [eq, eh, ef] now shows real values.
Per plan Task 0.7. Complete — no stubs.
0310b1d1eb