feat(jdk-codemodel): promote EnumConstantDescriptor to Traitable and stamp its declaration-site source location

[deer]

EnumConstantDescriptor used to only implement Trait, so unlike FieldDescriptor, FormalParameterDescriptor, and LambdaParameter, it had nowhere to attach a SourceLocation.FilePosition for the enum constant's declaration. This promotes it to also extend AbstractTraitable and implement Traitable, adding the @Marshal/@Unmarshal pair and Marshalling.register call needed for that, matching the structural shape of the earlier LambdaParameter promotion.

JdkInitializer.processMembers now calls addSourceLocation on each EnumConstantDescriptor it creates, the same way it already does for fields, methods, and parameters.

Also renamed LocationTraitTests to SourceLocationTests (and its test method names from ...ShouldCarryLocationTrait to ...ShouldCarrySourceLocation) since there is no LocationTrait type in the codebase -- the actual trait is SourceLocation, and the old name's javadoc reference to {@link LocationTrait} didn't resolve to anything.