feat(Radio): add activation prop for focus vs click selection

[johnleider]

Summary

Add an activation prop to Radio.Root matching the pattern established in Tabs.Root.

Background

The Tabs component introduced an activation prop with two modes:

• automatic (default): Selection occurs on focus (arrow key navigation)
• manual: Selection occurs on explicit Enter/Space key press

WAI-ARIA Research

Standard Behavior

Per WAI-ARIA APG, radio groups should have selection follow focus by default:

"When arrow keys move focus to a radio button, it simultaneously unchecks the previously focused button and checks the newly focused button."

This is the standard behavior, not optional like tabs.

Exception: Toolbar Context

Radio groups inside toolbars should NOT auto-select:

"When navigating through a radio group in a toolbar with arrow keys, the button that is checked does not change."

Rationale for Adding Prop

While automatic activation is the ARIA standard, providing activation="manual" allows:

• Toolbar radio group implementations
• User preference for deliberate selection
• Consistency with Tabs API

Proposed API

<Radio.Root v-model="selected" activation="manual">
  <Radio.Item value="a">Option A</Radio.Item>
  <Radio.Item value="b">Option B</Radio.Item>
</Radio.Root>

Implementation Notes

• Add activation prop to RadioRoot.vue with type 'automatic' | 'manual'
• Default to 'automatic' (standard ARIA behavior)
• Modify RadioItem.vue onFocus handler to check activation mode
• Add manual activation via Enter/Space in onKeydown

Industry Alignment

Library	Prop Name	Default
React Aria	keyboardActivation	'automatic'
Radix UI	activationMode	'automatic'
Vuetify v0	activation	'automatic'

Related

• Tabs implementation: packages/0/src/components/Tabs/TabsRoot.vue
• WAI-ARIA Tabs Pattern
• WAI-ARIA Radio Group Pattern

Checklist

• Add activation prop to RadioRoot
• Update RadioItem event handlers
• Add tests for both activation modes
• Update documentation