Add shuffle-lanes.css and --shuffle-index

[glen-cheney]

Phase 3 and 4 in #660

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This pull request adds and validates the staggered animation system for GridLanes items, ensuring smooth and accessible view transitions using CSS custom properties. It introduces the required stylesheet, implements the assignment and cleanup of the --shuffle-index property, and provides comprehensive tests to verify correct behavior and accessibility. Documentation is updated to reflect completed milestones and validation.

Staggered animation and CSS transition system:

• Added shuffle-lanes.css to define required view-transition selectors, keyframes, and custom property-based animation timing, including support for reduced-motion accessibility.
• GridLanes now assigns sequential --shuffle-index values to visible items for staggered transitions and removes them when items are hidden or disposed. [1] [2]

Testing and validation:

• Added unit and integration tests to verify correct assignment, removal, and effect of --shuffle-index, as well as validation of speed/easing options and compatibility with reduced motion. [1] [2]

Documentation updates:

• Marked animation, filtering, and sorting milestones as complete in the project checklist, reflecting that required CSS, logic, and tests have shipped. [1] [2] [3]