DI token every cngx interactive atom registers via useExisting: <self> so its host element doubles as a value-shape contract for
CngxFormBridge and any future Reactive-Forms / Signal-Forms
integration in @cngx/forms/controls.
No providedIn, no factory: providing the token is the atom's
responsibility. A surrounding consumer that wants a sane fallback
(e.g. an inert read-only value) supplies one in its own providers
block - the token deliberately does not invent a default because
"what is the value of an unbound control?" has no library-level
answer.