init: hgmj server initial commit

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

common/maths/sum.go

@@ -0,0 +1,25 @@
+package maths
+
+import "math"
+
+func Sum(input Float64Data) (sum float64, err error) {
+	if input.Len() == 0 {
+		return math.NaN(), EmptyInputErr
+	}
+	for _, n := range input {
+		sum += n
+	}
+	return sum, nil
+}
+
+func CumulativeInt64(a *int64, bs ...int64) *int64 {
+	var c int64
+	for _, b := range bs {
+		c += b
+	}
+	if a == nil {
+		return &c
+	}
+	*a += c
+	return a
+}