[feat]: Update <pf-v6-avatar> for PatternFly v6 #2979

Summary

Update `` to implement the PatternFly v6 Avatar component.

User profile image with size variants, border options, and fallback display.

Prior Art

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Avatar
• 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 `size`: `sm`, `md`, `lg`, `xl`, `2xl` - v6 adds 2xl, verify all sizes
• React `isBordered` boolean - map to `bordered` attribute
• `alt` attribute required on internal ``; surface as reflected attribute
• `src` attribute for image URL
• Consider fallback (e.g. initials or default SVG) when `src` missing or broken

Checklist

Rename

• Delete `elements/pf-v5-avatar/`
• 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`)