[feat]: Create <pf-v6-notification-badge> element #2937

Summary

Create `` to implement the PatternFly v6 Notification badge component.

Icon button with a count badge, indicating unread or attention-requiring notifications.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` NotificationBadge
• MAY adjust element API to leverage web platform strengths
• SHOULD maintain CSS custom property theming compatibility
• SHOULD expose useful CSS shadow parts

Proposed API

<!-- Default -->
<pf-v6-notification-badge count="0"></pf-v6-notification-badge>

<!-- Variants -->
<pf-v6-notification-badge variant="read | unread | attention" count="5"></pf-v6-notification-badge>

<!-- Expanded state (aria-expanded) -->
<pf-v6-notification-badge expanded count="10"></pf-v6-notification-badge>

Element-specific considerations

• React `variant`: `read`, `unread`, `attention` - drives color and icon
• React `count` - numeric badge; 0 may hide badge; map to `count` attribute
• React `isRead` (deprecated alias for `variant="read"`) - skip this alias
• React `onClick` - renders as a button; emit `click` natively
• `expanded` attribute sets `aria-expanded` via `ElementInternals` or reflected attribute - used when it toggles a notification drawer
• `aria-label` required for screen readers (e.g. "10 unread notifications")
• Integrates with `` as its trigger

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`)