[feat]: Create <pf-v6-checkbox> element #2574

Summary

Create `` to implement the PatternFly v6 Checkbox component.

Form checkbox input with label, description, and body content support.

Prior Art

Design

• PF spec

Notes

• Implement as a Form-Associated Custom Element (FACE) with shadow label via `label` attribute
• Use `ElementInternals` for form association and ARIA
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)
• Consider `referenceTarget` for label association (Chrome-only, progressive enhancement)
• Slots for body and description content, as well as controlled checkboxes
• It should be considered a violation to use a sibling `` along with the `label` attribute

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Checkbox
• MUST participate in native form submission via FACE
• 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 `isChecked` (controlled) / `defaultChecked` (uncontrolled) - reflect as `checked` attribute
• React `isIndeterminate` - maps to `indeterminate` property (not an HTML attribute); `ElementInternals` does not natively handle this - set on inner `` and expose as JS property
• React `description` - use `slot="description"`; React `body` - use default slot
• React `isLabelWrapped` - label text wraps vs truncates
• React `isDisabled` - propagates from `fieldset[disabled]` via `ElementInternals`
• `id`, `name`, `value` for form submission
• ARIA: `aria-checked` must reflect indeterminate state as `"mixed"`

Checklist

Implementation

• API per `.claude/ADVICE.md`
• CSS uses v6 tokens, logical properties, nesting
• `@summary` and JSDoc on element class
• `formAssociated = true` and `ElementInternals` for form participation
• Indeterminate state tested

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
• Form submission test

Reviews

• `/review-api` passes
• `/review-demos` passes
• `/review-a11y` passes
• `eslint` and `stylelint` pass

Ship

• Add changeset
• PR targets `staging/pfv6` (NOT `main`)