init: hgmj server initial commit

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

common/maths/mean.go

@@ -0,0 +1,11 @@
+package maths
+
+import "math"
+
+func Mean(input Float64Data) (float64, error) {
+	if input.Len() == 0 {
+		return math.NaN(), EmptyInputErr
+	}
+	sum, _ := input.Sum()
+	return sum / float64(input.Len()), nil
+}