feat(ui): add accessible tree suite

[EhabY]

Summary

• add the Tree, TreeItem, TreeGroup, and TreeIndentGuide component suite
• add controlled selection and expansion with ARIA tree semantics, roving tabindex, hierarchy navigation, and buffered type-ahead
• match VS Code 1.131 tree geometry and states, with separate shared, Modern, and stable styles
• add Storybook coverage across Light, Dark, High Contrast, and High Contrast Light
• use an observable useSyncExternalStore registry with batched reconciliation and per-item snapshots
• add focused component and keyboard-navigation tests

Stack

• Stacked on fix(ui): close package-wide gaps from the component library PRs #1047
• Base branch: feat/ui-package-gaps

Closes #1037

Validation

• pnpm test:webview — 357 tests passed
• pnpm typecheck
• NODE_OPTIONS=--max-old-space-size=4096 pnpm lint
• pnpm format:check
• pnpm storybook:ci
• pnpm build
• git diff --check

Pixel snapshots were not submitted locally because PIXEL_KEY was unavailable.

Implementation plan and decisions

API and state

• Compound hierarchy: Tree > TreeItem > TreeGroup > TreeItem
• Required stable itemId and textValue
• Controlled single selection through Tree.selectedItemId and Tree.onSelectedItemChange
• Controlled branch expansion through TreeItem.expanded and TreeItem.onExpandedChange
• No uncontrolled state or multi-selection API

Behavior

• Observable registry-based roving tabindex navigation with batched ordering and per-item snapshots
• Arrow Up/Down, Home/End, Left/Right, Enter/Space, and buffered type-ahead
• Enter and Space select the focused row and toggle branches
• Disabled and collapsed descendants are excluded; disabled rows cannot be programmatically activated
• Trailing interactive actions are isolated from tree selection and expansion
• Optional logical indexes and focus callbacks provide integration hooks for externally managed rows

Styling

• Tree.css contains shared structure and states
• Tree.modern.css contains the Modern UI row inset, radius, and keyboard-only focus behavior
• Tree.stable.css contains stable focus behavior; edge-to-edge square geometry follows from excluding Modern rules
• Modern UI is the package default; data-ui-style="stable" selects stable styling
• Semantic list and tree tokens map to VS Code theme variables
• Native 22px rows, 30px twistie allocation, 8px level offsets, and 16/24/32px indent guides

Coverage

• ARIA hierarchy, selection, expansion, disabled state, and accessible names
• Keyboard navigation, activation, type-ahead, controlled and metadata updates, and event isolation
• Stories across all four supported VS Code theme families, with stable variations only where they render differently from Modern

This PR was generated with Coder Agents.

Native tree visual audit

• extracted the VS Code 1.131 tree ground truth from the installed bundle (abstractTree.ts indent math, injected list styles, .style-override Modern rules) and verified against a live code serve-web explorer DOM: Modern rows are inset 4px with a 4px radius, guides sit at 16px + 8px per level, active guide owners derive from focus and selection, and inactive guides show only on hover
• the twistie gutter now follows the two native modes: the default keeps the 22px gutter on every row (for trees whose branch rows render icons), while variant="explorer" collapses the unused leaf gutter so file icons align with branch twisties, matching the native default Explorer whose folders render without icons — mixing folder icons with the collapsed gutter was what ran the active indent guide through leaf file icons
• the Tree story mirrors the native default Explorer (chevron-only folders, file icons on leaves), and guide visibility matches native: the active guide is always visible while inactive guides fade in only while the tree is hovered (verified against computed styles with a real pointer)
• simplified the store: per-item snapshots are read straight through the store subscription instead of a per-item observer registry, one registration path replaces registerElement plus the ready flag, the ancestor-path cache is gone, and the interactive-target selector is shared with TreeItem
• consolidated stories to stable variations only where Modern and stable differ (Tree, TreeItem), added collapsed-branch coverage, and made the Tree play reveal the row action through selection so it no longer depends on the window holding OS focus