fix(base-marshalling): support enum marshalling via registerEnum

[deer]

Enums cannot declare @Marshal/@Unmarshal methods, so there was previously no way to make isMarshallable return true for an enum class. This caused AbstractTraitable.destructor in codemodel to silently drop enum traits — they were filtered out by the marshaller.isMarshallable(trait.getClass()) guard with no error.

Adds SchemaFactory.registerEnum(Class<E>) (and Marshalling.registerEnum) as an explicit opt-in for enum classes, serializing by Enum.name() and deserializing via Enum.valueOf. Also fixes getMarshallingSchema and getUnmarshallingSchemas which were not consulting the enum registry, making them inconsistent with isMarshallable.