Integrate computed styles into layout and render pipeline #45

Description#

Update the layout and render crates to use computed styles from the style crate instead of hardcoded tag-based defaults. This is the final integration step for Phase 4.

Acceptance Criteria#

• Layout crate changes:
  • Remove hardcoded default_font_size(), default_margin(), display_type() functions
  • Read display, margin-*, padding-*, border-*-width, font-size, width, height from ComputedStyle
  • Resolve auto margins (centering)
  • Respect box-sizing: content-box (default) vs border-box
• Render crate changes:
  • Read color from computed style for text rendering
  • Read background-color for box backgrounds
  • Render borders with correct width and color
  • Support text-decoration: underline
• Browser crate changes:
  • Full pipeline: HTML → DOM → CSS parse → style resolve → layout → render → display
  • Both <style> and style="" attributes work
• All existing tests updated or replaced to work with the new style-driven layout
• Add integration tests: render an HTML page with <style> block and verify layout positions

Dependencies#

• All previous Phase 4 issues

Implementation Notes#

• This ties everything together
• Existing layout tests may need updating since they relied on hardcoded defaults
• The user-agent stylesheet in the style crate should produce equivalent results to the old hardcoded defaults for unstyled pages

Test milestone#

• After this issue, we should be on track toward Acid1 compliance