Extend readback_pinned from 3 to 16 floats and write Q-stats DtoH
into slots [3..8] instead of the non-pinned q_stats_pending field.
On H100 CUDA 13 driver, non-pinned host memory causes cuMemcpyDtoHAsync
to degrade to a synchronous copy, creating a latent hang risk.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>