Component-scoped configuration cascade for the sidenav family. Pass any
combination of feature factories in a parent component's viewProviders
array.
Unlike provideSidenavConfig (root-only), provideSidenavConfigAt injects
the parent injector's CNGX_SIDENAV_CONFIG value (resolves through the
priority chain - root provider, library defaults, or another
provideSidenavConfigAt further up) and deep-merges the supplied features on
top. Descendant sidenav instances see the merged config; sibling sub-trees
keep the inherited value untouched.