Purpose:
This package provides an overlay engine focused on anchored positioning,
lifecycle/close contracts, and deterministic policies (dismiss/focus/reposition).

Non-goals:
- No Material/Cupertino widgets or styling.
- No headless_* dependencies.
- No ready-made UI components.

Invariants:
- Overlay lifecycle must support closing phase and explicit completeClose.
- Policies must be deterministic and composable.
- Insertion uses Flutter primitives (OverlayPortal) without Navigator coupling.

Correct usage:
- Use as a runtime for menu/popover/tooltip/dialog overlays in any design system.

Anti-patterns:
- Mixing visual components into the engine.
- Coupling engine to app navigation or theme layers.
