reaction_target_unsupported· reason_code· applies to schema_validation, state_resolutionSub-reason for schema_violation when a ak.reaction.add / ak.reaction.remove target_ref points at an object kind that the deployment does not allow reactions on. v1 core only allows ak:message: targets; profiles MAY register additional target kinds. See zh/models/strand-and-message.md §9.8.2.