CngxBreadcrumbSiblingsSource

Contract an external sibling producer provides so CngxBreadcrumbSiblings reads its rows from a signal instead of the [siblings] input. The dropdown picks the source up through CNGX_BREADCRUMB_SIBLINGS_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()).

A producer such as the planned router-sync directive will provide this via useExisting; any producer exposing a Signal<readonly CngxBreadcrumbSibling[]> may provide it.