CloseNotifier mixin

Lets the lifecycle OWNER (core) react when a model built by a separate engine package closes. The package constructs the model; core registers a reset callback via addCloseListener; the concrete model fires the listeners from its own close()'s finally. Best practice: the engine createModel(spec, config) contract stays a pure factory — no lifecycle callback leaks into the contract that third-party engines implement.

Listeners fire EXACTLY ONCE provided close() is idempotent (if (_isClosed) return;). fireCloseListeners clears the list so a second invocation is a no-op even without the guard.