CngxBreadcrumbItemsSource

Contract an external crumb producer provides so CngxBreadcrumbBar reads its trail from a signal instead of the [items] input. The bar picks the source up through CNGX_BREADCRUMB_ITEMS_SOURCE and a computed that lets the source win over the input - the controlled path of the controlled/uncontrolled pattern (Pillar 1: no effect writes, no directive writing another component's input()).

The router-sync directive is the reference consumer; any producer that can expose a Signal<readonly CngxBreadcrumbCrumb[]> may provide it.