Fix for stale or overdrawn content in scrolled boxes

Claude Code draws its terminal screen by reusing parts already drawn and shifting them when you scroll. It now keeps track of clipped areas, the edges a box's contents are cut off at, and of content that draws past its own box. When a clipped area changes, it stops reusing or shifting the old drawing. This fixes stale or overdrawn content in boxes that are clipped and scrolled.