Restructure refimpl into go-lang and python subdirectories

Move Go reference implementation to refimpl/go-lang/ and add new
Python reference implementation in refimpl/python/. Update build.sh
with renamed draft and simplified tool paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

refimpl/go-lang/ect/audience.go

@@ -0,0 +1,34 @@
+package ect
+
+import "encoding/json"
+
+func marshalJSONString(s string) []byte {
+	b, _ := json.Marshal(s)
+	return b
+}
+
+func marshalJSONStringArray(a []string) []byte {
+	b, _ := json.Marshal(a)
+	return b
+}
+
+func unmarshalAudience(data []byte, a *Audience) error {
+	var raw json.RawMessage
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return err
+	}
+	if len(raw) > 0 && raw[0] == '[' {
+		var arr []string
+		if err := json.Unmarshal(raw, &arr); err != nil {
+			return err
+		}
+		*a = arr
+		return nil
+	}
+	var s string
+	if err := json.Unmarshal(raw, &s); err != nil {
+		return err
+	}
+	*a = []string{s}
+	return nil
+}