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>