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/create_test.go

@@ -0,0 +1,99 @@
+package ect
+
+import (
+	"crypto/ecdsa"
+	"encoding/json"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestCreateRoundtrip(t *testing.T) {
+	key, err := GenerateKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+	now := time.Now()
+	payload := &Payload{
+		Iss:         "spiffe://example.com/agent/a",
+		Aud:         []string{"spiffe://example.com/agent/b"},
+		Iat:         now.Unix(),
+		Exp:         now.Add(10 * time.Minute).Unix(),
+		Jti:         "e4f5a6b7-c8d9-0123-ef01-234567890abc",
+		Tid:         "550e8400-e29b-41d4-a716-446655440001",
+		ExecAct:     "review_spec",
+		Par:         []string{},
+		Pol:         "spec_review_policy_v2",
+		PolDecision: PolDecisionApproved,
+	}
+	compact, err := Create(payload, key, CreateOptions{KeyID: "agent-a-key-1"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if compact == "" {
+		t.Fatal("expected non-empty compact JWS")
+	}
+
+	// Verify with same key
+	resolver := func(kid string) (*ecdsa.PublicKey, error) {
+		if kid != "agent-a-key-1" {
+			return nil, nil
+		}
+		return &key.PublicKey, nil
+	}
+	opts := VerifyOptions{
+		VerifierID:  "spiffe://example.com/agent/b",
+		ResolveKey:  resolver,
+		Now:         now,
+		IATMaxAge:   15 * time.Minute,
+		IATMaxFuture: 30 * time.Second,
+	}
+	parsed, err := Verify(compact, opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if parsed.Payload.Tid != payload.Tid || parsed.Payload.ExecAct != payload.ExecAct {
+		t.Errorf("payload mismatch: got tid=%q exec_act=%q", parsed.Payload.Tid, parsed.Payload.ExecAct)
+	}
+}
+
+func TestCreateWithTestVector(t *testing.T) {
+	data, err := os.ReadFile("testdata/valid_root_ect_payload.json")
+	if err != nil {
+		t.Skipf("test vector not found: %v", err)
+		return
+	}
+	var p Payload
+	if err := json.Unmarshal(data, &p); err != nil {
+		t.Fatal(err)
+	}
+	key, err := GenerateKey()
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Override timestamps for verification
+	now := time.Now()
+	p.Iat = now.Unix()
+	p.Exp = now.Add(10 * time.Minute).Unix()
+
+	compact, err := Create(&p, key, CreateOptions{KeyID: "test-kid"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	resolver := func(kid string) (*ecdsa.PublicKey, error) {
+		if kid != "test-kid" {
+			return nil, nil
+		}
+		return &key.PublicKey, nil
+	}
+	_, err = Verify(compact, VerifyOptions{
+		VerifierID:   p.Aud[0],
+		ResolveKey:   resolver,
+		Now:          now,
+		IATMaxAge:    15 * time.Minute,
+		IATMaxFuture: 30 * time.Second,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+}