Per-step reduce_current_q_stats created GPU pipeline bubbles every 50 steps.
Each sync waited for the GPU to drain all pending graph replays before
the CPU could continue. Q-stats are now computed at epoch boundary only
(process_epoch_boundary already calls reduce_current_q_stats).
Training loop is now fully async: 178 graph launches queue in <1ms,
GPU executes back-to-back with zero pipeline stalls.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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