- Flip --gpu-eval default to true (--no-gpu-eval to opt out) - Move actions_history scatter-write into env kernel (zero CPU accumulation) - Remove done-flag periodic download (env kernel handles per-thread) - Only GPU→CPU transfer is final metrics readback (n_windows × 40 bytes) - Add spread cost discrepancy warning when GPU path is active Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>