Migrates the 3 clone_htod_f32 calls in walk-forward GPU data upload (features, targets, OFI) to mapped_pinned::clone_to_device_f32_via_pinned. Audit row appended (Fix 6) in docs/dqn-gpu-hot-path-audit.md. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>