[feat]: Update `<pf-v6-background-image>` for PatternFly v6

[bennypowers]

Summary

Update `` to implement the PatternFly v6 Background image component.

Full-page background image with optional filter overlay, typically used behind login pages.

Prior Art

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

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` BackgroundImage
• 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 `src` accepts a single string URL or a `BackgroundImageSrc` object (`xs`, `xs2x`, `sm`, `sm2x`, `lg` breakpoints) for responsive images; decide whether to support both or just single URL via attribute
• React `filter` - CSS filter string applied as an overlay (e.g. blurred gradient); defaults to built-in SVG filter
• Element is purely decorative - must set `aria-hidden="true"`
• Injects an SVG `` element into the page; verify shadow DOM approach vs. light DOM injection
• No interactive behavior; no focusable children

Checklist

Rename

• Delete `elements/pf-v5-background-image/`
• Update `elements/package.json` exports
• Update `eslint.config.js` ignore globs if applicable
• Update docs `_snippets/` and `framework-integration/` references

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`)