Injection token that resolves the factory used to instantiate a
SelectionController. Defaults to createSelectionController;
override app-wide via providers: [{ provide: CNGX_SELECTION_CONTROLLER_FACTORY, useValue: customFactory }]
or per-component via viewProviders to inject cross-cutting concerns.
Symmetrical to CNGX_SELECT_COMMIT_CONTROLLER_FACTORY in @cngx/forms/select —
same pattern, applied at the selection-primitive level so future
@cngx/data-display grid/tree components share the override surface.