Purpose:
- Headless TextField for Flutter (behavior + a11y), delegating visual rendering to theme capabilities.

Non-goals:
- No baseline Material/Cupertino UI is provided in this package.
- No app-level state management, formatting, validation, or business rules.

Invariants:
- Rendering is delegated to `RTextFieldRenderer` (theme capability).
- Missing renderer capability throws a standardized `[Headless] Missing required capability...` error.
- Semantics are owned by the component; renderer must not duplicate the root semantics.
- Controlled/uncontrolled follows Flutter conventions: external controller is not disposed.

Style sugar:
- RTextFieldStyle is convenience sugar converted to `RenderOverrides.only(RTextFieldOverrides.tokens(...))`.
- Priority: explicit overrides > style > theme defaults.

Correct usage:
- Wrap your app/subtree with `HeadlessThemeProvider` and provide `RTextFieldRenderer`.

Anti-patterns:
- Importing concrete renderer implementations into this package.
- Creating implicit visual defaults inside `RTextField`.

