init: hgmj server initial commit

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

common/maths/deviation.go

@@ -0,0 +1,34 @@
+package maths
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+)
+
+func StandardDeviationPopulation(input Float64Data) (sdev float64, err error) {
+	if input.Len() == 0 {
+		return math.NaN(), EmptyInputErr
+	}
+	vp, _ := PopulationVariance(input)
+	return math.Pow(vp, 0.5), nil
+}
+
+func DivideInt64(a, b int64) float64 {
+	if b == 0 {
+		return 0
+	}
+	return float64(a) / float64(b)
+}
+
+func DivideFloat64(a, b float64) float64 {
+	if b == 0 {
+		return 0
+	}
+	return a / b
+}
+
+func ToFloat64_b2(v float64) float64 {
+	f, _ := strconv.ParseFloat(fmt.Sprintf("%.2f", v), 64)
+	return f
+}