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

[bennypowers]

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

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

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`)