Discriminated-union shape returned by the sidenav config features -
withSidenavDimensions, withSidenavResponsive, withSidenavShortcut,
withSidenavHoverDwell, and withSidenavRouterSync. The reducer in
provideSidenavConfig / provideSidenavConfigAt matches on kind and merges
payload into the corresponding config sub-tree: dimensions, hover, and
routerSync are one-level nested sub-trees, responsive and shortcut are
flat top-level scalars. Mirrors CngxBreadcrumbConfigFeature from
@cngx/ui/breadcrumb (nested-sub-tree style; the stepper family uses flat
scalar keys instead).