feat: Generic `Spec` type support

Hi, it seems like we don't support exporting a generic Spec type, currently Spec looks like this:

interface Spec {
    /** Root element key */
    root: string;
    /** Flat map of elements by key */
    elements: Record<string, UIElement>;
    /** Optional initial state to seed the state model.
     *  Components using statePath will read from / write to this state. */
    state?: Record<string, unknown>;
}

However, UIElement is a fully generic type:

interface UIElement<T extends string = string, P = Record<string, unknown>> {
    /** Component type from the catalog */
    type: T;
    /** Component props */
    props: P;
// ...

I wonder if we could add support for a Spec<T, P> type as well. For context, I want to pass Spec as a type to a React Hook Form form, but the type of elements should be limited to my own catalog only, and I have no way to that now without redefining Spec myself.

Thank you.

Start by locating the exported Spec and UIElement type definitions and checking how Spec is consumed by the React Hook Form integration. Define the generic relationship between the element catalog and props while preserving existing Spec usage; done means consumers can constrain elements to their own catalog without redefining Spec.