Function interval_stats_exact 

fn interval_stats_exact<'a>(
    units: &'a [Unit],
    extents: &[Extent],
) -> IntervalStats<'a>

Totals of an interval without materializing split_conflicts.

After close_units, files of different units never conflict, so only split units can conflict across units, and each split unit holds one file. The dense fallback may instead treat every file as a split unit, and then the same pairwise loop covers every cross-unit pair. Counting them takes O(K²) for an interval of at most K files, instead of building a window-wide adjacency.