import dataset rows

Import a structured host result by column mapping, without retyping numbers. result: {columns: [...], rows: [[...]]} or {rows: [{...}]}. mapping: {entity_column|entity_constant, period_column, metrics: [{metric, column, unit}], basis, source_type, source_url, source_title, rows_path?}. Evidence is retained, every value is gated, identical retries are idempotent. Task remains open for more batches;