Add clip rect support to the rendering pipeline so that boxes with overflow != visible clip their children's content to the padding box. Render crate changes: - Add PushClip/PopClip variants to PaintCommand - Maintain a clip rect stack in Renderer that intersects nested clips - Apply active clip rect in fill_rect, composite_glyph, and draw_image - overflow:hidden, overflow:auto, and overflow:scroll all clip content - overflow:visible (default) continues to render without clipping Layout crate fix: - Fix is_empty_block to not treat blocks with explicit CSS height as empty (they should not self-collapse per CSS2 §8.3.1) Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>