Function pick_small_l1 

pub(crate) fn pick_small_l1(
    files: &[FileHandle],
    mapper: &PrimaryKeyRangeMapper,
    limits: PickLimits,
) -> Option<UnitPick>

Reduces small L1 files left behind by a closed merge, including in settling windows. The input bytes fit in one output and no input dominates its peers, so each merge removes files without repeatedly rewriting an accumulator.

Large and L0 files remain in the external conflict index. Check only the best reducing interval to bound planning work; rejecting it can miss other admissible intervals, but cannot increase the window’s conflict count.