Replace the GPU->CPU sync barrier (to_vec1 readback) in the DQN training
hot path with the GpuTrainingGuard CUDA kernel that performs NaN detection,
loss clipping, and gradient collapse checks entirely on-device. The guard
is lazy-initialized on first training step and falls back to the original
CPU readback path if CUDA kernel compilation fails.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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