Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add scrollable overflow regions for overflow:scroll and overflow:auto
boxes, with rendered scroll bars and mouse wheel event support.

Layout crate:
- Add content_height field to LayoutBox tracking natural content height
before CSS height override (used for overflow detection)
- Reserve SCROLLBAR_WIDTH (15px) from content area for overflow:scroll
- Export SCROLLBAR_WIDTH constant

Render crate:
- Add ScrollState type (HashMap<NodeId, (f32, f32)>) for scroll offsets
- Modify display list builder to accept scroll state and translate offset
- Apply scroll offset to child coordinates in scrollable containers
- Render vertical scroll bars (track + proportional thumb) for
overflow:scroll (always) and overflow:auto (when content overflows)
- Add page-level scroll support via paint_with_scroll()
- Add build_display_list_with_page_scroll() for viewport scrolling
- 6 new tests: scrollbar rendering, scroll offset, thumb proportionality,
overflow:auto conditional scrollbar, page scroll

Platform crate:
- Add scrollWheel: event handler to WeView class
- Add set_scroll_handler() for registering scroll callbacks
- Forward scroll delta (dx, dy) and mouse position to handler

Browser crate:
- Add page_scroll_y, content_height, scroll_offsets to BrowserState
- Handle scroll wheel events: update page scroll, clamp to bounds,
re-render
- Pass scroll state through rendering pipeline

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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>