DI token a submenu-companion directive provides so composers can resolve
its CngxMenuSubmenuLike contract via inject(CNGX_MENU_SUBMENU_ITEM)
from within the submenu subtree.
Note: the surrounding CngxMenu does NOT discover submenus through this
token. Discovery is DI registration via
CngxMenuHost.registerSubmenuItem (which crosses component view
boundaries a content query cannot), and the result is exposed on
CngxMenuHost.submenuItems.