Function compute_describe_single_aggregation 

pub fn compute_describe_single_aggregation(
    df: &DataFrame,
    schema: &Schema,
    total_rows: usize,
    sample_size: Option<usize>,
    sample_seed: u64,
    polars_streaming: bool,
) -> Result<AnalysisResults>

Computes describe statistics in a single aggregation pass over the DataFrame. Uses one collect() with aggregated expressions for all columns (count, null_count, mean, std, min, percentiles, max).