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

[bennypowers]

Summary

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

Panel for displaying notification history, typically opened from the page header. Supports grouped notifications and read/unread states.

Prior Art

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

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` NotificationDrawer
• 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 `isGrouped` - notifications grouped by category; map to `grouped` attribute
• React sub-components: `NotificationDrawerHeader`, `NotificationDrawerBody`, `NotificationDrawerGroup`, `NotificationDrawerList`, `NotificationDrawerListItem`, `NotificationDrawerListItemHeader`, `NotificationDrawerListItemBody` - use named slots
• `NotificationDrawerGroup`: `count`, `isExpanded`, `isRead`, `onExpand`, `title` - accordion-like group headers
• `NotificationDrawerListItem`: `variant` (`success`, `warning`, `danger`, `info`, `default`), `isRead`, `isHoverable`, `onClick`
• Header has "Mark all read" action and close button
• Often rendered inside a `` (uses it as the panel)
• Notifications should use `aria-live` at the container level for dynamic updates

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`)