feat: improve structure of ref and sys tokens; add useFocusVisible hook

[adrcotfas]

Motivation

Foundation prep for the Switch component modernization.

1. Add focus-indicator tokens at md.sys.state.focusIndicator (thickness: 3, outerOffset: 2). Matches MD3 spec. Switch will be the first consumer.
2. Move misplaced tokens from ref to sys per the MD3 spec:
   - ref.stateOpacity -> sys.state.opacity
   - ref.scrimAlpha -> sys.scrim.alpha
3. Split tokens/index.ts (was 364 LOC, mixed ref values + sys decisions + barrel).
4. add useFocusVisible hook: required to detect when to show a focus ring

Also in this PR:

• New helper getStateLayer(theme, role, state) in theme/utils/state.ts. Returns { color, opacity }. Four components migrated: HelperText, TextInput/Adornment, CheckboxItem, RadioButtonItem.
• New types ColorRole = keyof ThemeColors and StateOpacityKey = keyof typeof tokens.md.sys.state.opacity. Component token files can use them to validate string keys.
• theme/types/utils.ts moved to src/types.tsx. The helpers there ($Omit, $RemoveChildren, EllipsizeProp) are not theme-specific.

No change to Theme. Renames are internal only.

Related issue

N/A. Prep for Switch modernization.

Test plan

• yarn typescript clean
• yarn lint clean
• yarn test green, snapshots stable

[github-actions]

The mobile version of example app from this branch is ready! You can see it here.

[callstack-bot]

Hey @adrcotfas, thank you for your pull request 🤗. The documentation from this branch can be viewed here.