Function compute_data_quality_watched 

pub fn compute_data_quality_watched(
    lf: &LazyFrame,
    total_rows: Option<usize>,
    plan: &DataQualityPlan,
    source: Option<&QualitySourceContext>,
    polars_streaming: bool,
    kept: Option<&QualitySample>,
    watch: &QualityWatch,
) -> (Result<DataQualityResults>, Option<QualitySample>)

The data quality of lf by plan, cutting kept instead of reading when it serves the plan. Names each stage to watch and stops between stages (or inside a streamed read) once cancelled. A sampled run’s rows come back even if it stopped: the read is paid for, and the next run can cut it.