Step 5b: Layout engine — inline layout and text wrapping

[thomasnemer]

Parent: #24
Prerequisites: Step 5a

Goal

Implement inline formatting context: line box construction, text wrapping, text-align, baseline alignment.

Implementation

• Line box construction: accumulate inline boxes, break when exceeding container width
• Text wrapping: break at whitespace boundaries (greedy)
• white-space: normal (collapse+wrap), nowrap, pre, pre-wrap
• text-align: left, center, right
• Inline box splitting across lines
• Mixed block+inline: anonymous block wrapping

Tests

• Short text one line, long text wraps, nowrap, text-align center
• Multiple inline elements on same line
• ~8 tests

Acceptance Criteria

• cargo test -p ie-layout passes
• Clippy clean