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

[bennypowers]

Summary

Create `` to implement the PatternFly v6 Menu toggle component.

Toggle button that opens and closes a menu. Supports plain, default, primary, and split button variants.

Prior Art

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

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` MenuToggle
• 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`, `plain`, `plainText`, `primary`, `secondary` - map to `variant` attribute
• React `isExpanded` - drives `aria-expanded` and chevron direction; map to `expanded` attribute
• React `isFullWidth`, `isFullHeight` - layout attributes; map to boolean attributes
• React `isDisabled` - disables the toggle button
• React `icon` - leading icon (slot); `badge` - notification badge (slot)
• React `status`: `success`, `warning`, `danger` - status icon + color variant
• Split button variant: `splitButtonOptions` - renders a checkbox/action button beside the toggle arrow; complex, verify v6 scope
• `aria-expanded` must be set; `aria-haspopup` should reflect menu type (`"menu"`, `"listbox"`, `"tree"`, etc.)
• Chevron icon rotates on `isExpanded`; use CSS transform

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`)