- do-back: uses steps-to-preview + render-to-dom (O(1) render) instead of replaying every do-step from 0 (O(n) DOM ops + cssx) - Hydration effect: same rebuild-preview instead of step replay - Cookie save moved to button on-click only (not per-step) Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>