[feat]: Create <pf-v6-expandable-section> element #2932

Summary

Create `` to implement the PatternFly v6 Expandable section component.

Toggle-controlled content area that shows or hides a section of content.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` ExpandableSection
• 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 `variant`: `default`, `truncate`, `detached` - note: detached and truncated are NOT required in initial implementation (see original issue notes)
• React `isExpanded` (controlled) / `defaultExpanded` (uncontrolled) - reflect as `expanded` attribute
• React `isDetached` - toggle and content are separate sibling elements, not parent/child; complex pattern
• React `isWidthLimited` - constrains content width to a percentage
• React `isIndented` - adds left margin to content
• React `displaySize`: `default`, `lg`
• React `toggleText`, `toggleTextExpanded`, `toggleTextCollapsed` - text on toggle button; map to single `toggle-text` + optional `toggle-text-expanded` / `toggle-text-collapsed` attributes
• Consider native `
  `/`

  ` as inner implementation (handles toggle semantics natively), though it may limit CSS animation of content reveal
• Emit `toggle` event on expand/collapse

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`)