Purpose:
Cupertino preset/theme + renderers package for Headless components.

Non-goals:
- No component state logic.
- No dependency on other component packages.

Invariants:
- Provides capability-based renderers through theme composition.
- Keeps API Flutter-like and stable (v1 decisions).
- Overrides priority (v1.1): Cupertino*Overrides > R*Overrides > theme defaults.
- Popover visuals (blur/shadow/opacity) приходят из tokens; opacity отдельным полем.

Correct usage:
- Use with HeadlessThemeProvider / presets to render R* components with Cupertino look.
- Use CupertinoButtonOverrides/CupertinoDropdownOverrides only for Cupertino-specific knobs.

Anti-patterns:
- Putting overlay/listbox logic here (must stay in headless_foundation).

