[feat]: Create `<pf-v6-form-select>` element

Start by reading .claude/ADVICE.md and comparing the PatternFly v6 Form select design with @patternfly/react-core FormSelect. Define the custom element API and native option/optgroup slotting approach from the issue requirements, then cover the listed public API, accessibility, and form-submission tests. Done means visual and feature parity, passing review and lint checks, a changeset, and a PR targeting staging/pfv6.

Summary

Create `` to implement the PatternFly v6 Form select component.

Styled native select dropdown for simple single-selection form inputs.

Prior Art

Notes

• Implement as a Form-Associated Custom Element (FACE) using `ElementInternals`
• Use ARIA IDL attributes for cross-root ARIA (Baseline 2025)
• Consider `referenceTarget` for label association (Chrome-only, progressive enhancement)

Requirements

• MUST implement PFv6 visual designs
• MUST provide end-user feature parity with `@patternfly/react-core` FormSelect
• MUST participate in native form submission via FACE
• 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

• This is a styled native `` - not the custom floating Select; simpler and more accessible React `validated`: `default`, `success`, `warning`, `error` - drives border/icon; `error` sets `aria-invalid` React `isDisabled` - pass through to inner ``
• React `isIconSprite` - uses CSS background image for the chevron; verify v6 behavior
• React `FormSelectOption` and `FormSelectOptionGroup` - consider `` and `` sub-elements, OR allow slotting native `` / `` directly (preferred for simplicity)
• Custom styling of native `` is limited; use `appearance: none` + CSS arrow Slotting native `` is the most composable approach and best for screen readers Checklist 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