HasCenterScale
Result trait for accumulators that expose a center estimate and a scale estimate. Consumed by the band operator (which derives center ± k * scale) and by the Standardize AST node (which projects (x - center) / scale).
Implemented by com.eignex.kumulant.stat.summary.WeightedVarianceResult (center = mean, scale = stdDev), com.eignex.kumulant.stat.summary.MomentsResult (same shape), com.eignex.kumulant.stat.summary.MadResult (center = median, scale = MAD), and com.eignex.kumulant.stat.summary.SummaryResult. A consumer asking for HasCenterScale doesn't need to know which provenance the center/scale came from.