feat: Support sections in GridLayout

[mehdibha]

Closes

Currently you have to choose between virtualization and sections in a grid: GridLayout treats every top level collection node as a fixed size cell, so section and header nodes are never walked and a sectioned GridList/ListBox cannot be virtualized with <Virtualizer layout={GridLayout}>. ListLayout supports sections (headingSize/estimatedHeadingSize), but not grid arrangement.

This gap shows up in any sectioned grid: emoji pickers (I hit this building one in dotUI — a GridList with layout="grid", ~1,900 GridListItems grouped into GridListSections with GridListHeaders, filtered with Autocomplete), photo galleries grouped by album/date, dashboards grouped by category.

This PR adds section support to GridLayout:

• Section nodes are walked like in ListLayout: the layout produces a hierarchical LayoutInfo for each section (parents before children, so CollectionBranch rendering and isPersistedKey ancestor checks work), sized to contain its rows.
• Header nodes are laid out as full width rows interleaved with the uniform grid rows. Each section starts on a new row.
• New GridLayoutOptions: headingSize and estimatedHeadingSize, mirroring ListLayout's modern option names (no deprecated *Height variants). Without headingSize, headers are measured, starting from estimatedHeadingSize (default 48) like ListLayout.
• Headers of visible sections are always included in getVisibleLayoutInfos (matching ListLayout's behavior, e.g. for sticky positioning), and loaders inside sections are laid out as full width rows.
• getDropTargetFromPoint ignores section/header rows when searching for drop targets.
• Keyboard navigation across section boundaries works with the existing ListKeyboardDelegate grid logic because columns stay x-aligned across sections (covered by tests).

Backward compatibility: flat collections produce the same layout as before. update() was restructured from a precomputed rows loop into a streaming walk, preserving the existing behaviors: loader nodes still consume a grid slot and the trailing loader sentinel is laid out at the end; skeleton nodes still repeat to fill the remainder of the row and then the viewport; empty collection and preserveAspectRatio/measured-height semantics are unchanged. All existing GridLayout/GridList/ListBox tests pass unchanged, and the only public API change is the two new optional options.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

• yarn jest packages/react-stately/test/virtualizer/GridLayout.test.ts — new geometry tests for sections/headers (plus the existing drop target tests).
• yarn jest packages/react-aria-components/test/GridList.test.js — new virtualized grid layout with sections tests covering rendering/virtualization of offscreen sections and keyboard navigation across section boundaries (ArrowDown/Up keep the column across a section header, End reaches the last offscreen item).
• Storybook: GridList → VirtualizedGridListGridSection — 10 sections × 25 items in a virtualized grid with headers. Scroll, click and arrow-key around; compare with the existing VirtualizedGridListGrid (flat) story for unchanged behavior.

🧢 Your Project:

dotUI