[feat]: Create <pf-v6-menu> element #3020

Summary

Create `` to implement the PatternFly v6 Menu component.

Versatile list of actions, links, or options. Used as the basis for dropdown, select, and other menu-based components.

Prior Art

Sub-components

• `` - Individual action, link, or checkbox/radio item
• `` - Grouped items with optional title
• `` - List container within a group

Requirements

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

• Foundation for dropdown, select, nav flyout - port early; other elements depend on it
• React `isNavFlyout` - flyout variant with arrow keys to open sub-menus
• React `isPlain` - removes box shadow/border
• React `isScrollable` - scrollable menu with fixed height
• React `isGrouped` - contains `MenuGroup` children
• React `activeItemId` (controlled) / `onSelect` - selection state; emit `select` event
• React `onActionClick` - secondary action on `MenuItemAction` (the right-side icon button on an item)
• `MenuItem` props: `itemId`, `isDisabled`, `isExternalLink`, `description`, `icon`, `badge`, `to` (href), `flyoutMenu`
• `MenuItemAction` - secondary interactive element on an item (e.g. a bookmark icon)
• `MenuSearch` - search input above the menu list
• Keyboard: Arrow keys navigate items, Enter/Space select, Escape closes, right arrow enters flyout
• ARIA: `role="menu"`, items have `role="menuitem"` (or `"menuitemcheckbox"`, `"menuitemradio"`)

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`)