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/rolling_window.go

@@ -0,0 +1,17 @@
+package datascience
+
+// RollingWindow genera ventanas deslizantes de tamaño size sobre el slice xs.
+// Si size <= 0 o size > len(xs), retorna nil.
+func RollingWindow[T any](xs []T, size int) [][]T {
+	n := len(xs)
+	if size <= 0 || size > n {
+		return nil
+	}
+	windows := make([][]T, 0, n-size+1)
+	for i := 0; i <= n-size; i++ {
+		w := make([]T, size)
+		copy(w, xs[i:i+size])
+		windows = append(windows, w)
+	}
+	return windows
+}