Function pick_closed_merge_by_class 

pub(crate) fn pick_closed_merge_by_class(
    files: &[FileHandle],
    mapper: &PrimaryKeyRangeMapper,
    options: ClosedMergeOptions,
) -> [Option<UnitPick>; 2]

Discovers file reduction and overlap resolution together, sharing both the conflict graph and interval traversal. The first slot is file reduction.