feat(checkbox): modernize for md3 spec compliance

Rewrites the Checkbox renderer to match the Material Design 3 spec
(https://m3.material.io/components/checkbox/specs):

- 18dp container with 2dp outline (unselected) / 0dp outline + theme
  primary fill (selected), inside a 40dp state-layer tap target.
- State-layer overlay renders hover (8%), focus (10%) and pressed (10%)
  layers in the color the spec defines for each (selected pressed flips
  to onSurface; error always wins).
- Focus indicator: 3dp ring at theme.colors.secondary with the 2dp
  outer-offset from md.sys.state.focusIndicator. Gated on
  :focus-visible via the useFocusVisible hook added in #4952.
- Animations approximate Compose Material3 Checkbox.kt: 100ms fill
  transition and 150ms checkmark draw, sequenced short-leg then
  long-leg to suggest the stroke fraction. Indeterminate uses a
  scaleX-animated dash.
- No new peer-deps: the checkmark is built from two rotated rectangles
  (View-based), not an SVG path.

utils.ts:
- New getSelectionVisualState helper returns the full color +
  opacity + outline-width picture for a given state combo.
- Legacy getSelectionControlColor kept as a compatibility export
  for RadioButtonAndroid (radio button modernization is out of
  scope for this PR).

9 snapshots auto-updated to reflect the new render tree.

Author: fabriziocucci