Add WIMSE ECT reference implementation (Go)

- ect library: create, verify, DAG validation, ledger interface
- In-memory ledger and ECTStore for full ledger mode
- Test vectors and unit tests; two-agent demo (cmd/demo)
- README: document refimpl scope and usage

Co-authored-by: Cursor <cursoragent@cursor.com>

refimpl/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
+}