DiagStaging runs DtoD copies on a separate CUDA stream — zero stalls
on the training pipeline. Double-buffered mapped-pinned: host reads
previous step's data while current step's copies run concurrently.
Replaces 7 blocking read_slice_d calls (ISV, rewards, dones, actions,
raw_rewards, trade_duration, outcome_ema) with async staging reads.
One-step delay on diag data (acceptable for diagnostics).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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