Reuse build_units/unit_correctness (pair contract single entry) in the
inference aggregation step: single questions count as one unit, AR pairs
collapse original+mirror into one unit scored by bidirectional AND. total/
correct/per_task_type are unit-grained; orphan pairs (missing one side) are
warned and dropped, not counted. Per-question predictions still land row by
row (traceability unchanged).
iomgaa