CustomRadio: option to deselect the selected item by pressing it again

[kemuru]

Version: 3.9.0 — src/lib/form/custom-radio.tsx (and radio-group.tsx)

Some groups need to be clearable by pressing the selected option again. kleros-v2's dispute feature selection is one:
choosing a feature disables incompatible features in the other group, so users must be able to undo a choice
without a separate control (kleros/kleros-v2#2281, web/src/components/DisputeFeatures/Features/FeatureRadio.tsx).

There is no supported way to do this from a consumer today:

• The library bundles its own react-aria; its useRadio reads only value, children, aria-label,
  aria-labelledby, so onPress/onPressStart on CustomRadioItem are ignored.
• The label press selects in the click bubble phase and stops propagation, and the consumer's own
  RadioGroupStateContext (from its react-aria-components) is a different module instance.
• What works is a capture-phase click/keyup handler that defers the clear with setTimeout(0) so it lands after
  react-aria's press. It is a timing workaround, and a test in jsdom cannot even detect when it breaks.

Proposal: allowDeselect?: boolean on CustomRadio (and Radio). Inside the group, intercept the press on an
item whose value === state.selectedValue and call state.setSelectedValue(null) (react-aria's
RadioGroupState.setSelectedValue accepts null), so onChange(null) reaches the consumer. Space on the selected
item should behave the same. Radios are not normally deselectable, so an explicit opt-in keeps the default semantics.