fn pick_closed_merge_candidates(
files: &[FileHandle],
mapper: &PrimaryKeyRangeMapper,
options: ClosedMergeOptions,
work_budget: usize,
per_class: bool,
) -> [Option<UnitPick>; 2]fn pick_closed_merge_candidates(
files: &[FileHandle],
mapper: &PrimaryKeyRangeMapper,
options: ClosedMergeOptions,
work_budget: usize,
per_class: bool,
) -> [Option<UnitPick>; 2]