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/clip.go

@@ -0,0 +1,17 @@
+package datascience
+
+// Clip recorta cada valor del slice para que quede dentro del rango [min, max].
+func Clip(data []float64, min, max float64) []float64 {
+	result := make([]float64, len(data))
+	for i, v := range data {
+		switch {
+		case v < min:
+			result[i] = min
+		case v > max:
+			result[i] = max
+		default:
+			result[i] = v
+		}
+	}
+	return result
+}