CngxTocContract

Minimal contract a toc rail exposes to its collaborators (the opt-in CngxTocRouterSync fragment directive): the active-section signal it derives, the activation output it emits, and the imperative scrollTo it owns. Injected via CNGX_TOC + useExisting, never by injecting the concrete CngxToc class - that would create a cyclic type dependency and block the decompose brain/skin split.

Direction mirrors CNGX_SIDENAV, not CNGX_BREADCRUMB_ITEMS_SOURCE: the rail owns the state (activeId, scrollTo) and the collaborator consumes it, so the toc provides the token and the sync directive injects it.