Purpose:
- Headless Checkbox components for Flutter (RCheckbox, RCheckboxListTile) with behavior + a11y.

Non-goals:
- No renderer implementation in this package.
- No preset-specific visuals or design tokens.

Key types (public API):
- RCheckbox: Controlled checkbox (value + onChanged).
- RCheckboxListTile: Checkbox with row layout and slots.
- RCheckboxStyle: Simple style sugar -> overrides.
- RCheckboxListTileStyle: Simple style sugar -> overrides.

Style sugar:
- RCheckboxStyle / RCheckboxListTileStyle are convenience sugar converted to
  `RenderOverrides.only(RCheckboxOverrides.tokens(...))` and
  `RenderOverrides.only(RCheckboxListTileOverrides.tokens(...))`.
- Priority: explicit overrides > style > theme defaults.

Slots:
- RCheckboxSlots and RCheckboxListTileSlots use SlotOverride (Replace/Decorate/Enhance).
- Prefer Decorate-first to preserve default structure and semantics.

Invariants:
- Rendering is delegated to renderer capabilities from HeadlessTheme.
- Single activation source: component owns pointer/keyboard activation.
- Semantics are owned by the component; renderers must not duplicate root semantics.

Correct usage:
- Provide RCheckboxRenderer and RCheckboxListTileRenderer through HeadlessTheme.

Anti-patterns:
- Implementing visuals directly inside component widgets.
