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

[bennypowers]

Summary

Create `` to implement the PatternFly v6 Radio component.

Form radio button with label, description, and body content support.

Prior Art

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

Design

• PF spec

Notes

• Implement as a Form-Associated Custom Element (FACE) using `ElementInternals`
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)
• Consider `referenceTarget` for label association (Chrome-only, progressive enhancement)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Radio
• 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

• `id`, `name`, `value` attributes are required for proper radio group behavior and form submission
• React `isChecked` (controlled) / `defaultChecked` (uncontrolled) - reflect as `checked` attribute
• React `description` - secondary text below label; use `slot="description"`
• React `body` - additional content below description; use default slot or `slot="body"`
• React `isLabelWrapped` - label text wraps vs truncates
• React `isDisabled` - disable via `ElementInternals`; propagates from `fieldset[disabled]`
• Radio grouping: browsers handle mutual exclusion via shared `name`; FACE must also implement this
• ARIA: inner `` or `role="radio"` with `aria-checked`; label via `aria-labelledby`

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
• Radio group mutual exclusion 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`)