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Adds the fourth and final 4-cases panel: KV split S_kv-wise across the 8 cubes (cube=row_wise, S_local = S_kv/C), replicated within each cube (pe=replicate). PE-TP at B=1 means only PE 0 of each cube has work; PEs 1-7 early-return (slide-11 PE-TP-at-B=1 waste). No intra-CUBE comm; inter-CUBE 8-way reduce reuses the lrab-adapted center-root pattern (root cube 6) — same structural cost as Case 4's inter-CUBE phase (21 ipcq_copy). 16 tests pass (4 per case). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>