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

Start by reading .claude/ADVICE.md and the existing elements/pf-v5-select/ implementation, then study the PatternFly v6 Select source linked in the issue. Review elements/package.json, eslint.config.js, and the documented _snippets/ and framework-integration/ references before defining the new element structure. Done means the v6 component, sub-components, demos, accessibility and form tests, reviews, changeset, and staging/pfv6 delivery checklist are complete.

Summary

Update `` to implement the PatternFly v6 Select component.

Custom selection dropdown supporting single-select, multi-select, typeahead filtering, and grouped options.

Prior Art

Sub-components

• `` - Individual selectable option
• `` - Grouped options with title

Notes

• Consider implementing as a FACE for form submission
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` Select
• 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

• MAJOR API CHANGE: React v6 Select is rebuilt on `Menu` + `MenuToggle` architecture - fundamentally different from v5. Study v6 React source before porting.
• React v6 `Select` props: `toggle` (render prop), `isScrollable`, `isGrouped`, `selected`, `onSelect` - manages selected state externally
• `SelectOption`, `SelectList`, `SelectGroup` are thin wrappers around Menu equivalents
• Single-select vs multi-select: no separate component - parent manages `selected` array
• Typeahead: not built into Select in v6 - use `TextInputGroup` inside the toggle; implement filtering in JS
• FACE: MUST use `ElementInternals` for `name`/`value` form submission; multi-select must serialize all selected values
• Consider `` as a slottable child element (like native ``)
• ARIA: `role="listbox"`, options have `role="option"` + `aria-selected`; toggle has `aria-haspopup="listbox"` + `aria-expanded`

Checklist

Rename

• Delete `elements/pf-v5-select/`
• 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
• `formAssociated = true` and `ElementInternals` for form participation
• Form submission tested (single and multi-select)

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
• Form submission test

Reviews

• `/review-api` passes
• `/review-demos` passes
• `/review-a11y` passes
• `eslint` and `stylelint` pass

Ship

• Add changeset
• PR targets `staging/pfv6` (NOT `main`)