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>)

compute_data_quality_kept, naming each stage to watch as it enters it and stopping between stages, or inside a streamed read, once watch is cancelled.

The sample a sampled run read comes back whether or not the run finished: a run stopped after its read has still paid for it, and the next run can cut it.