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

[bennypowers]

Summary

Update `` to implement the PatternFly v6 Tabs component.

Tabbed content panels with horizontal or vertical orientation, supporting default, box, and filled variants.

Prior Art

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

Sub-components

• `` - Individual tab trigger
• `` - Content panel associated with a tab

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Tabs
• 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`: `default`, `box`, `vertical`, `horizontal` - and combinations like `box vertical`; verify v6 variant names
• React `isBox`, `isVertical`, `isSecondary`, `isFilled` - map to boolean attributes
• React `activeKey` (controlled) vs `defaultActiveKey` (uncontrolled) - reflect as `active-key` attribute
• React `onSelect` - fires with tab key and event - emit `tab-select` or `change` custom event
• React `mountOnEnter`, `unmountOnExit` - lazy content mounting; may not apply to web component slots
• React `inset`, `usePageInsets` - padding tokens
• React `overflow menu` - tabs that overflow collapse into a dropdown; complex, verify v6 scope
• ARIA: `role="tablist"`, tabs have `role="tab"` + `aria-selected` + `aria-controls`; panels have `role="tabpanel"` + `aria-labelledby`
• RTI (Roving Tab Index) controller for keyboard navigation (← → arrow keys)

Checklist

Rename

• Delete `elements/pf-v5-tabs/`
• 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`)

[bennypowers]

Consider consolidating the API, see if we can make API/A11Y Gains:

<pf-v6-tabs>
  <pf-v6-tab name="TabName" icon="foo">Tab panel content here</pf-v6-tab>

instead of have pf-tab/pf-tab-panel pairs