Function eval_bitmap_query_bucket_iter 

pub fn eval_bitmap_query_bucket_iter<'a, S>(
    source: S,
    query: BitmapQuery,
    range: Range<u64>,
    bucket_size: u64,
    direction: ScanDirection,
) -> impl Iterator<Item = BitmapScanResult<Watermarked<(u64, RoaringBitmap)>>> + 'a
where
    S: BitmapBucketIteratorSource<'a>,

Evaluate a DNF BitmapQuery as an ordered iterator of marked bucket bitmaps. Output emits Watermarked::Item((bucket_id, bitmap)) interleaved with Watermarked::Watermark(p) derived from the slowest leaf’s progress.