[feat]: Create <pf-v6-input-group> element #3015

Summary

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

Groups form inputs with addons (buttons, text, selects) into a visually connected unit.

Prior Art

Sub-components

• `` - Individual item within the group

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` InputGroup
• 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 `InputGroupText`: `variant`: `plain` or `box` - label/addon text beside inputs
• React `InputGroupItem`: `isFill` (grows to fill), `isBox`, `isPlain`, `isDisabled`
• Pure layout element; connects child elements visually with shared border and no gap
• CSS approach: flex container; inner items share border-radius only at outer edges (use `:first-child`/`:last-child` selectors)
• No interactive behavior on the group itself; children handle their own interaction

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`)