Purpose:
This package provides shared behavior primitives used by multiple Headless components:
focus policies, listbox navigation, interactions, state resolution, effects executor.

Non-goals:
- No UI baselines / no Material-like visuals.
- No dependency on headless_theme (foundation must not depend on theme).

Invariants:
- The package is stable infrastructure; avoid breaking changes.
- Keep the package independent from app navigation (no Navigator/Route coupling).

Correct usage:
- Components use these primitives to implement behavior and expose state to renderers/themes.

Anti-patterns:
- Putting “shared widgets” here instead of behavior mechanisms.
- Adding dependencies to component packages or headless_theme.

