Step 5c: Layout engine — positioned layout

[thomasnemer]

Parent: #24
Prerequisites: Step 5b

Goal

Implement CSS positioned layout: static, relative, absolute, fixed.

Implementation

• position: static — normal flow (default)
• position: relative — offset from normal position
• position: absolute — removed from flow, relative to positioned ancestor
• position: fixed — relative to viewport
• Stacking context tracking (z-index)

Tests

• Relative offset, absolute positioning, fixed positioning
• ~5 tests

Acceptance Criteria

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