ae942f6959
Adds the third 4-cases panel: KV replicated per cube (8× memory waste), PEs SP on S_kv within each cube, intra-CUBE 8-way reduce on (m, ℓ, O), and no inter-CUBE comm (every cube ends with full answer; designated writer = cube 0). Reuses the row-chain + col-bridge intra-CUBE pattern that anchors Case 4 (21 ipcq_copy per cube × 8 cubes = 168 total). 12 tests pass (4 Case 4 + 4 Case 2 + 4 Case 3). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>