[feat]: Create `<pf-v6-tree-view>` element

[bennypowers]

Summary

Create a <pf-v6-tree-view> web component implementing the PatternFly v6 Tree view component.

Hierarchical tree display for nested data, with expandable nodes, checkboxes, and action menus.

Prior Art

Source	Element	Status
PFE v4	--	--
cem	<pf-v6-tree-view>	Exists
cockpit	<pf-v6-tree-view>	Exists
chickadee	--	--

Sub-components

• <pf-v6-tree-item> - Individual tree node

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with @patternfly/react-core TreeView
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Element-specific considerations

• Tree ARIA pattern: role="tree" on root, role="treeitem" per node, role="group" on child lists
• aria-expanded on expandable nodes, aria-selected for selection
• Keyboard: Arrow Up/Down navigate siblings, Arrow Right expands/enters children, Arrow Left collapses/exits, Home/End jump to first/last, Enter/Space select
• hasCheckboxes variant: each node has checkbox; aria-checked for tri-state (indeterminate when partial children selected)
• hasActions variant: action button per node (e.g., edit, delete)
• hasBadges variant: badge on each node showing child count
• hasGuides variant: vertical guide lines
• isNested for compact nested style
• onSelect event with node data; onExpand / onCollapse
• activeItems prop for controlled selection
• Search/filter: onSearch callback or toolbar integration
• Data-driven API: data prop array vs slotted <pf-v6-tree-item> children - evaluate both approaches

Checklist

Implementation

• API per .claude/ADVICE.md
• CSS uses v6 tokens, logical properties, nesting
• @summary and JSDoc on element class

Demos

• Demos match patternfly.org naming/structure
• CSS custom properties match computed styles from patternfly.org (Chrome MCP)
• Visual parity verified via Chrome MCP

Tests

• Public API covered (attributes, properties, events, slots)
• a11ySnapshot assertions

Reviews

• /review-api passes
• /review-demos passes
• /review-a11y passes
• eslint and stylelint pass

Ship

• Add changeset
• PR targets staging/pfv6 (NOT main)