Inline layout: inline boxes, line boxes, text wrapping #44

Description#

Implement proper inline layout in the layout crate, replacing the current simplified text wrapping with spec-compliant inline formatting context.

Implement inline formatting context (IFC):
- Inline boxes: generated by inline-level elements (<span>, <a>, <em>, <strong>, etc.)
- Line boxes: horizontal lines that contain inline boxes and text runs
- Anonymous inline boxes: for bare text not wrapped in an inline element
Text runs within inline boxes:
- Break text into words at whitespace boundaries
- Wrap lines when content exceeds containing block width
- Handle <br> as a forced line break
Vertical alignment within line boxes:
- vertical-align: baseline (default)
- Line box height determined by tallest content
- Baseline alignment of text across different font sizes
Inline box model:
- Inline elements can have margin, padding, border (left/right applied, top/bottom don't affect line height)
- Background and borders render across line breaks (split across lines)
Text alignment (text-align):
- left (default), center, right, justify
Line height (line-height):
- Normal (default ~1.2x font size)
- Number (multiplier)
- Length (px, em)
Replace hardcoded layout defaults with computed styles from the style crate
Write tests for multi-line wrapping, inline elements with padding, text alignment, mixed font sizes

The current layout crate has basic text wrapping — this replaces it with proper inline layout
Key change: layout reads from ComputedStyle instead of hardcoded tag-based defaults
This is the most complex layout task in Phase 4