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

[bennypowers]

Summary

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

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

Prior Art

Source	Element	Status
PFE v4	--	--
cem	--	--
cockpit	--	--
chickadee	--	--

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 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`)