Function aggregate_attr 

pub fn aggregate_attr(
    tile: &SparseTile,
    attr_idx: usize,
    reducer: Reducer,
) -> AggregateResult

Reduce one attr column over all rows of a tile. attr_idx must be in range; non-numeric / null cells are skipped (Count includes non-null cells regardless of dtype, since “count of cells with this attribute populated” is a sensible fold across all attr types).