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/impute.md

@@ -0,0 +1,21 @@
+---
+name: impute
+kind: function
+lang: go
+domain: datascience
+version: "1.0.0"
+purity: pure
+signature: "func Impute(data []float64) []float64"
+description: "Rellena valores NaN en un slice de float64 usando forward-fill, reemplazando cada NaN con el último valor válido anterior."
+tags: [datascience, impute, missing, fill]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: ["math"]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "functions/datascience/impute.go"
+---