Function dense_universe_buckets 

pub fn dense_universe_buckets(
    range: Range<u64>,
    bucket_size: u64,
    direction: ScanDirection,
) -> impl Iterator<Item = (u64, RoaringBitmap)> + Send + 'static

Synthesized bucket sequence for the dense tx-seq universe: one full bitmap per bucket touched by range, in scan-direction order. Backends return this for the query-only IndexDimension::TxUniverse key instead of reading storage — the tx-seq namespace is dense, so the universe is computable.

Bitmaps carry all [0, bucket_size) relative bits even in edge buckets; trimming against the requested range happens at the flatten step, same as for stored buckets.