feat: 15 funciones datascience — estadística, DSP e IO de datos

12 funciones puras con implementación real:
Standardize, MinMaxScale, Clip, RollingWindow, ZipSlices, GroupBy,
Histogram, Pearson, Autocorrelation, FFT (Cooley-Tukey), DetectOutliers, Impute

3 funciones impuras (stubs):
LoadCSV, LoadParquet, FetchDataFrame

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

functions/datascience/group_by.go

@@ -0,0 +1,11 @@
+package datascience
+
+// GroupBy agrupa los elementos de un slice según la clave devuelta por keyFn.
+func GroupBy[T any, K comparable](xs []T, keyFn func(T) K) map[K][]T {
+	groups := make(map[K][]T)
+	for _, x := range xs {
+		k := keyFn(x)
+		groups[k] = append(groups[k], x)
+	}
+	return groups
+}