Replace 3 read_slice_d calls (each allocs+frees MappedF32Buffer) and
4 read_scalar_d calls with pre-allocated persistent isv_staging,
frd_loss_staging, and scalar_staging buffers. Deletes the now-unused
read_scalar_d function.
Eliminates ~20 cuMemHostAlloc + ~20 cuMemFree per step from the hot
path. GPU completes all kernels in 160us/step — host overhead was 14ms.
Target: <1ms host overhead at b=256.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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