087
blvh:slot:087sampling_operation; slot=087; candidate_set=control; subgroup=clean_singleton; target_period=diversify 1840-1899; query_pool=; mask_rule=prefer exactly 1 instance with score >=0.80 and clean boundary; book_cap=max 2 selected images per book; selection_method=crop_masks structural filter + diversity review; required_provenance=fname, record_id, bbox, score, area, rectangularity, model_id, model_sha; status=TO_SELECT; sampling_version=blvh-0.2. observation_status=machine_selection_plan; NEVER promote the semantic query to a factual subject label.