Function pick_active_l1_with_budget 

fn pick_active_l1_with_budget(
    l1_files: &[FileHandle],
    mapper: &PrimaryKeyRangeMapper,
    options: PickLimits,
    work_budget: usize,
) -> Option<UnitPick>

pick_active_l1 with an explicit work budget, so tests can exercise the single-file fallback without a window-sized conflict graph.