Function compute_chunk_partial 

pub fn compute_chunk_partial(
    bytes: &[u8],
    dtype: &DType,
    fill_value: Option<&FillValue>,
) -> (Option<StatValue>, Option<StatValue>, u64, u64)

Computes (min, max, null_count, row_count) for a single chunk’s raw bytes.

null_count counts elements equal to fill_value; 0 when none is set. Floats use total_cmp so NaN sorts last (max). FixedSizeList / List return (None, None, 0, 0).