Per spec §5.4 point 2. heads_w_skip storage 5× reduced (640→128 floats). Each horizon reads only its bucket via offset lookup. Block-tree reduction (padded to 32 lanes for clean halving stride), no atomicAdd. GPU oracle test matches naive per-horizon computation (tol 1e-5) on sm_86. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>