- Remove ALL agent.forward() from DQN training hot paths
- Replace per-step Q-divergence check with cuBLAS compute_q_stats
- Remove dead test_training_rejects_missing_gpu_collector (tests dead code)
- Add Drop impls: FusedTrainingCtx syncs stream + drains errors before drop
- GpuDqnTrainer Drop: drain deferred CUDA errors via check_err()
- Sequential GPU test contamination: cudarc in-process context limitation,
run ignored GPU tests via separate cargo invocations (CI already does this)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>