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

Start with .claude/ADVICE.md and the PatternFly v6 Empty state reference linked in the issue. Define the element API, demos, and tests around the listed attributes, named slots, accessibility snapshots, and visual requirements, then use the /review-api, /review-demos, and /review-a11y checks plus eslint and stylelint to verify completion.

Summary

Create `` to implement the PatternFly v6 Empty state component.

Placeholder content displayed when no data is available, with icon, title, body text, and action slots.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` EmptyState
• 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`: `xs`, `sm`, `lg`, `xl`, `full` - drives icon and text sizes; map to `size` attribute
• React `status`: `custom`, `danger`, `info`, `success`, `warning` - drives icon color; map to `status` attribute
• React `headingLevel`: `h1`-`h6` for the title; map to `heading-level` attribute
• React `titleText` - the title; use `slot="title"` or `title` attribute
• React `icon` - icon component; use `slot="icon"` (defaults to `` with status icon)
• `EmptyStateBody`, `EmptyStateActions`, `EmptyStateFooter` - use named slots
• Loading variant: swap icon slot for `` to show loading empty state
• `full` variant fills the container's full height (100%)

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`)