Add a retained layer tree system that tracks layout box state between frames and computes damage rectangles for partial repainting. The system compares box fingerprints (position, color, opacity, text content, borders) across frames to identify which screen regions changed, merges nearby damage rects to reduce draw calls, and skips rendering entirely when nothing changed. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>