CngxSidenavContract

Minimal contract a sidenav rail exposes to the router-sync directive: the two-way opened signal it reads and writes. Injected via CNGX_SIDENAV

• useExisting, never by injecting the concrete CngxSidenav class - that would create a cyclic type dependency and block the decompose brain/skin split. The deferred layout-coordination consumer will broaden this contract (with close() / elementRef) in the branch that migrates CngxSidenavLayout onto the token.