feat(react): add the Radio primitive

[ivanbanov]

Summary

Adds the Radio primitive as two packages:

• @dunky.dev/radio — the framework-free core: a radio-group state machine built on @dunky.dev/state-machine, following the WAI-ARIA APG Radio Group pattern. Owns value state (controlled and uncontrolled), item registration, focus/roving-tabindex decisions, and disabled gating.
• @dunky.dev/react-radio — the thin React binding exposing the Radio compound component; parts carry the ARIA wiring and data-state / data-disabled attributes, styling stays with the consumer.

Anatomy

<Radio>                    - root; owns the checked value, renders nothing of its own
  |_ <Group>               - the radio group surface; carries the group semantics
     |_ <Item>             - one option; checked or unchecked, one item tabbable at a time
     |  |_ <ItemIndicator> - the visual checked mark; exists only while its item is checked
     |_ <ItemLabel>        - names its item; pressing it selects the item

Behavior

• Controlled (value + onValueChange) and uncontrolled (defaultValue) checked value; null means no selection.
• Roving tabindex: exactly one item tabbable at a time; arrow keys walk enabled items from the focused one, wrapping at both ends and skipping disabled items, and selection follows focus.
• Space checks a focused unchecked item (the tab-in case); Enter deliberately does nothing, per the APG, so implicit form submission isn't triggered.
• role="radiogroup" / role="radio" with aria-checked, aria-orientation from the orientation hint, and per-item label association via ItemLabel (wired by id, never leaving a dangling reference).
• Disabled gating at group and item level: disabled items carry aria-disabled and leave the tab order (not native disabled), staying perceivable to assistive tech.
• ItemIndicator renders only while its item is checked — presence is the styling contract, on top of data-state="checked" / "unchecked".
• Form integration (hidden inputs) is intentionally out of scope for v0.

Test plan

• pnpm test:ci — 131 tests, 9 files, all passing
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm build

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