cuda_if_available() silently fell back to CPU when CUDA init failed,
causing DQNTrainer to crash later with "cuda_stream() called on CPU
device". This was the root cause of the CI "hang" — the test failed
immediately but the error was invisible due to buffered output.
- DQNTrainer::new() now uses MlDevice::cuda(0)? with explicit error
- cuda_if_available() now logs the CUDA error before falling back
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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