Opt-in h3_vsa_fp4.sp_input_gather (default false; requires sp_projection_overlap) gathers the
already-packed FP4 inputs and rotary tables, then computes each rank's assigned Q/K/V heads locally;
the gate projection overlaps the smaller transfer. Supported on the measured GB10/cuDNN shape only;
other shapes keep the existing exchange. Enabled in the Spark-pair example.
T2AV on 2 Sparks: 19.35 -> 18.76 s/clip (merge gate check, same SP reduction order rules).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
