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

[bennypowers]

Summary

Create `` to implement the PatternFly v6 Navigation component.

Application sidebar or horizontal navigation with support for grouped items, expandable sections, and flyout menus.

Prior Art

Source	Element	Status
PFE v4	--	--
cem	``	Exists
cockpit	``	Exists
chickadee	``	Exists

Sub-components

• `` - Individual navigation link
• `` - Grouped navigation items with title
• `` - Expandable section within navigation

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Nav
• 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

• React `variant`: `default`, `horizontal`, `tertiary`, `horizontal-subnav` - map to `variant` attribute
• React `isScrollable` - vertical nav with scroll; map to `scrollable` attribute
• React `aria-label` - required for `` landmark (e.g. "Global navigation")
• `NavItem`: `to` (href), `isActive`, `component`, `icon`, `flyoutMenu` (for flyout menus)
• `NavGroup`: `title`, `id`, `aria-label`
• `NavExpandable`: toggle for sub-nav; isExpanded, defaultExpanded, onExpand; contains `NavList`
• Flyout menus: `NavItem flyoutMenu` - opens a `` on hover/arrow key; needs Floating UI
• Active item: `aria-current="page"` on the active `NavItem` link
• Wraps in `` element for landmark

Checklist

Implementation

• API per `.claude/ADVICE.md`
• CSS uses v6 tokens, logical properties, nesting
• `@summary` and JSDoc on element class
• Renders `` element with `aria-label`

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`)