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

[bennypowers]

Summary

Update <pf-v6-avatar> to implement the PatternFly v6 Avatar component.

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

Prior Art

Source	Element	Status
PFE v4	<pf-avatar>	Exists
cem	--	--
cockpit	--	--
chickadee	<chickadee-avatar>	Exists

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