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/dag_test.go

@@ -0,0 +1,61 @@
+package ect
+
+import (
+	"testing"
+	"time"
+)
+
+func TestValidateDAG_Root(t *testing.T) {
+	store := NewMemoryLedger()
+	payload := &Payload{
+		Tid:         "task-001",
+		Wid:         "wf-1",
+		Par:         []string{},
+		PolDecision: PolDecisionApproved,
+	}
+	err := ValidateDAG(payload, store, DefaultDAGConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestValidateDAG_DuplicateTid(t *testing.T) {
+	store := NewMemoryLedger()
+	// Pre-insert same tid
+	_, _ = store.Append("dummy-jws", &Payload{Tid: "task-001", Wid: "wf-1", Par: []string{}, PolDecision: PolDecisionApproved})
+	payload := &Payload{Tid: "task-001", Wid: "wf-1", Par: []string{}, PolDecision: PolDecisionApproved}
+	err := ValidateDAG(payload, store, DefaultDAGConfig())
+	if err == nil {
+		t.Fatal("expected error for duplicate tid")
+	}
+}
+
+func TestValidateDAG_ParentExists(t *testing.T) {
+	store := NewMemoryLedger()
+	_, _ = store.Append("jws1", &Payload{Tid: "task-001", Wid: "wf-1", Par: []string{}, PolDecision: PolDecisionApproved, Iat: time.Now().Unix() - 60})
+	payload := &Payload{
+		Tid:         "task-002",
+		Wid:         "wf-1",
+		Par:         []string{"task-001"},
+		PolDecision: PolDecisionApproved,
+		Iat:         time.Now().Unix(),
+	}
+	err := ValidateDAG(payload, store, DefaultDAGConfig())
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestValidateDAG_ParentNotFound(t *testing.T) {
+	store := NewMemoryLedger()
+	payload := &Payload{
+		Tid:         "task-002",
+		Par:         []string{"task-missing"},
+		PolDecision: PolDecisionApproved,
+		Iat:         time.Now().Unix(),
+	}
+	err := ValidateDAG(payload, store, DefaultDAGConfig())
+	if err == nil {
+		t.Fatal("expected error when parent not found")
+	}
+}