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

[bennypowers]

Summary

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

Group of mutually exclusive or multi-selectable toggle buttons.

Prior Art

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

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`)