[Epic] Tree Grid & Hierarchical Data Support

Goal

Implement native, high-performance support for Tree Grids and hierarchical data structures within the Neo.mjs framework. This Epic focuses on the core "Hierarchical Rows" pattern (a flat virtualized view of tree nodes) ensuring O(1) rendering performance and strict adherence to the Row Pooling architecture.

Scope & Architecture

The Tree Grid implementation is designed to integrate seamlessly with the existing Neo.grid.Body virtual scroller without requiring variable-height rows.

1. Data Abstraction (Neo.data.TreeStore & TreeModel): The complexity of the hierarchy is entirely managed by the data layer. The TreeStore parses hierarchical data (using parentId relations) into an internal O(1) #childrenMap. Crucially, it exposes a dynamically updated, flattened 1D array of only the visible nodes to the grid. When a node is expanded, its children are injected into this flat array.
2. Visual Representation (Neo.grid.column.Tree): A specialized, nested column component handles the visual state. It utilizes CSS variables (--tree-depth) for dynamic indentation math and CSS classes for toggle icon states, ensuring rapid recycling during scroll events without DOM manipulation.
3. Accessibility (WAI-ARIA): By leveraging the flattened array approach, the virtual scroller naturally provides gapless, contiguous aria-rowindex values. The implementation is augmented with aria-expanded, aria-level, aria-setsize, and aria-posinset attributes mapped directly from the records (utilizing "Soft Hydration" where necessary).
4. Interaction: Expansion and collapse interactions are handled via event delegation on the Grid Body, triggering state mutations in the Store, which subsequently fire events (expand, collapse) for View Controller consumption.

Note: Variable-height "Row Expanders" (nested content bodies) are outside the scope of this Epic and would require a separate architectural enhancement to the ScrollManager.