feat: modernize RadioButton for MD3

[burczu]

Motivation

RadioButton still shipped as two platform-specific implementations (RadioButtonAndroid, RadioButtonIOS) with the iOS variant rendering a checkmark glyph rather than a Material radio. This is inconsistent with the rest of the v6 MD3 work (Checkbox, Switch, TextInput, FAB) and diverges from the M3 radio button spec.

This PR unifies RadioButton into a single MD3 component that mirrors the modernized Checkbox structure (single component, tokens.ts, error prop, accessible={false} inner control), without pulling in Checkbox's reanimated/focus-ring machinery (out of scope — radio ships with none today).

Related issue

Closes #4938

What changed

• Unified component — merged RadioButtonAndroid + RadioButtonIOS into a single MD3 RadioButton (Android impl as base). Dropped RadioButton.Android, RadioButton.IOS, RadioButtonIOS, and the RadioButton.Item mode prop.
• Tokens — new RadioButton/tokens.ts (MD3 dims 20/10/2 + color roles); colors resolved via tokens in utils.
• error prop — added to RadioButton and RadioButton.Item (ring + dot use theme.colors.error; disabled/custom colors take precedence).
• Single animation path (reanimated) — collapsed two RN Animated.Values into one shared value and migrated to react-native-reanimated (matching Checkbox): dot scales in with overshoot on selection, keyed on checked so it also animates inside a RadioButton.Group. Uses theme.motion MD3 duration/easing tokens and respects reduce-motion.
• Disabled opacity — the control now dims to 38% (selectionControlOpacity was previously computed but never applied).
• Group perf — memoized the provider value + stabilized onValueChange with use-latest-callback; consumers switched from Context.Consumer to useContext.
• Item a11y — inner control is accessible={false} + container importantForAccessibility="no-hide-descendants", so a screen reader sees one radio per row; dropped per-radio accessibilityLiveRegion; labelMaxFontSizeMultiplier=1.5 + ${testID}-text for CheckboxItem parity.
• Docs/examples — updated examples (Group/Item, leading/trailing, error checked+unchecked, disabled); removed the iOS-mode demo and the RadioButtonAndroid/RadioButtonIOS doc pages.

Breaking changes

• Removed RadioButton.Android, RadioButton.IOS, RadioButtonIOS (and RadioButtonAndroidProps/RadioButtonIOSProps exports).
• Removed the RadioButton.Item mode prop.
• error prop added (no visual change for existing usage).

Test plan

• yarn typescript, yarn lint, yarn test (snapshots updated) — green.
• yarn --cwd docs build — green.
• Manually verified on iOS sim, Android emulator, and web: single selection, dot scale-in (standalone and in a Group), error (checked/unchecked), disabled dimming, leading/trailing, ripple, single a11y node per Item row.

Videos

radio_ios.mp4

radio_android.mp4

radio_web.mp4