Root cause: GpuTensor::cat() called to_host() on each input tensor,
concatenated on CPU, then from_host() back to GPU. This caused:
1. Race conditions when async GPU work hadn't completed before to_host
2. CUDA_ERROR_ILLEGAL_ADDRESS when event tracking was disabled
3. Massive performance hit (2 DMA transfers per tensor per cat call)
Fix: replace with DtoD memcpy via CudaSlice::slice() views. Both dim=0
(contiguous blocks) and dim>0 (interleaved rows) use GPU-to-GPU copies.
Zero CPU involvement, zero event dependency, zero race conditions.
Also: revert global disable_event_tracking() — was a workaround for the
to_host race, no longer needed with GPU-native cat/stack.
5/5 DQN pipeline tests pass with CUDA Graph enabled on RTX 3050.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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