feat: implement CSS order property for flex and grid items

[jacquesg]

Objective

Implement the CSS order property for flex and grid items, as defined in CSS Display Module Level 3 §3.

This allows users to control the visual ordering of flex and grid children independently of their source (DOM) order, which is a commonly expected CSS feature that taffy was missing.

Context

The CSS order property is an integer (default 0) that controls:

• Flexbox (§5.4): The order in which flex items appear within their flex container during layout.
• CSS Grid (§6.3): The order-modified document order used during auto-placement.

Items with lower order values are laid out first. Items with equal order values preserve their source order (stable sort).

Changes

• Added order: i32 field to Style (gated behind flexbox or grid features)
• Added order() method to both FlexboxItemStyle and GridItemStyle traits (defaults to 0 for backwards compatibility)
• Flexbox: After generating flex items, stable-sort them by order and reassign visual indices
• Grid: Sort children by order before auto-placement, assign visual order before track sizing
• Added order field to GridItem to carry visual order through to final positioning
• Comprehensive tests covering reordering, source-order stability, and negative values for both flex and grid

Feedback wanted

The order property is defined once in CSS Display Level 3 but applies to both flex and grid contexts. I added order() to both FlexboxItemStyle and GridItemStyle traits separately
(each defaulting to 0) rather than pulling it into CoreStyle. This avoids a larger trait refactor while keeping backwards compatibility for custom style implementations. Open to feedback on whether a shared trait approach would be preferred.