fix(codemodel-foundation): marshal AnnotationValue.Value variants as real registered types

[deer]

AnnotationValue.Value (Literal, ClassRef, EnumConstant, Nested, Array) was marshalled through hand-rolled toValue/fromValue helpers that converted each variant to a raw Object for serialization. The conversion was asymmetric: ClassRef serialized to a bare TypeName and EnumConstant serialized to a formatted "Type.CONSTANT" string, but the deserialize direction (toValue) had no matching cases for either, so both silently came back as a Value.Literal after an unmarshal, losing their real type and, for EnumConstant, the type name and constant name as separate fields.

Each Value variant is now its own registered marshalled record, using @Unmarshal/@Marshal methods and Marshalling.register, the same pattern PersonRecord already demonstrates in base-marshalling's own test suite for a plain record with no Trait/Composite involved. AnnotationValue itself now marshals value as a plain Marshalled<Value> instead of routing through the raw-Object helpers, which are removed entirely.

The bug was never caught because the only marshalling round-trip test touching AnnotationValue (MarshallingTests#shouldMarshallAndTransportAndUnmarshallAnnotationTypeUsage) only ever exercised Value.Literal, the one variant where the raw-Object round-trip happened to work. Added dedicated round-trip tests for the other four variants.