- Add z_index field to ComputedStyle and parse CSS z-index property
- Remove absolute/fixed elements from normal flow (block, inline, flex layout)
- Position absolute elements relative to nearest positioned ancestor's padding box
- Position fixed elements relative to viewport
- Support left/right/top/bottom offsets with percentage resolution
- Support width/height stretching when both sides are specified
- Implement stacking context paint order: negative z-index, in-flow, non-negative z-index
- Update normalize_children and margin collapsing to exclude out-of-flow elements
- Add tests for absolute positioning, fixed positioning, z-index, stretching, and
bottom/right positioning

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

Per CSS Box Alignment §8, row-gap applies between rows and column-gap
between columns. In a column flex container the main axis is vertical,
so the gap between items should use row-gap (not column-gap) and the
gap between flex lines should use column-gap (not row-gap).

Introduces main_gap/cross_gap variables that swap based on flex direction.

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

Add full flexbox support with flex container and item properties:

CSS parsing:
- Parse display: flex/inline-flex
- Parse flex container props: flex-direction, flex-wrap, justify-content,
align-items, align-content, gap/row-gap/column-gap
- Parse flex item props: flex-grow, flex-shrink, flex-basis, align-self, order
- Expand flex, flex-flow, and gap shorthands

Layout algorithm (CSS Flexbox Level 1 §9):
- Determine main/cross axes from flex-direction
- Content-based flex-basis sizing for auto basis
- Distribute free space via flex-grow/flex-shrink
- Line wrapping (flex-wrap: wrap/wrap-reverse)
- Main axis justification (all justify-content modes)
- Cross axis alignment (all align-items/align-self modes)
- Multi-line align-content distribution
- Visual ordering via order property
- Gap spacing between items
- Reverse direction support (row-reverse, column-reverse)

Tests: 10 new flex layout tests covering row/column direction,
grow/shrink, justify-content, align-items, wrap, gap, and order.

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