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

Start by reading .claude/ADVICE.md and reviewing the referenced @patternfly/react-core ToggleGroup along with the existing cem, cockpit, and chickadee elements. Establish the element and item API, visual styles, demos, accessibility behavior, and tests from the checklist. Done means the implementation passes the listed reviews, lint checks, tests, and includes a changeset targeting staging/pfv6.

Summary

Create `` to implement the PatternFly v6 Toggle group component.

Group of mutually exclusive or multi-selectable toggle buttons.

Prior Art

Sub-components

• `` - Individual toggle button

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` ToggleGroup
• 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 `isCompact` on group - reduced spacing; map to `compact` attribute
• React `areAllGroupsDisabled` - disables entire group; map to `disabled` attribute
• `ToggleGroupItem`: `text`, `icon`, `isSelected`, `onChange`, `isDisabled`, `buttonId`, `aria-label`
• Single-select vs multi-select: React defaults to multi-select (each item independently toggleable); consider a `single` attribute for exclusive-select mode
• RTI (Roving Tab Index) controller for keyboard navigation
• ARIA: `role="group"` on container with `aria-label`; items are ``

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`)