Function pick_closed_merge_with_budget 

fn pick_closed_merge_with_budget(
    files: &[FileHandle],
    mapper: &PrimaryKeyRangeMapper,
    options: ClosedMergeOptions,
    work_budget: usize,
) -> Option<UnitPick>

pick_closed_merge with an explicit work budget, so tests can exercise the dense fallback without building a window-sized conflict graph.