feat: migrate refimpls from draft-00 to draft-01 claim names

- Rename `par` to `pred` (predecessor) in types, serialization, tests
- Remove `pol`, `pol_decision` from core payload; move to `ect_ext`
- Remove `sub` from payload (not part of ECT spec)
- Update `typ` from `wimse-exec+jwt` to `exec+jwt` (accept both)
- Rename MaxParLength to MaxPredLength everywhere
- Update testdata, demos, READMEs with migration table
- All Go tests pass, all 56 Python tests pass (90% coverage)

refimpl/go-lang/cmd/demo/main.go

@@ -27,16 +27,18 @@ func main() {
 
 	// 1) Agent A creates root ECT (task id = jti per spec)
 	payloadA := &ect.Payload{
-		Iss:         agentA,
-		Aud:         []string{agentB},
-		Iat:         now.Unix(),
-		Exp:         now.Add(10 * time.Minute).Unix(),
-		Jti:         "550e8400-e29b-41d4-a716-446655440001",
-		Wid:         "wf-demo-001",
-		ExecAct:     "review_requirements_spec",
-		Par:         []string{},
-		Pol:         "spec_review_policy_v2",
-		PolDecision: ect.PolDecisionApproved,
+		Iss:     agentA,
+		Aud:     []string{agentB},
+		Iat:     now.Unix(),
+		Exp:     now.Add(10 * time.Minute).Unix(),
+		Jti:     "550e8400-e29b-41d4-a716-446655440001",
+		Wid:     "wf-demo-001",
+		ExecAct: "review_requirements_spec",
+		Pred:    []string{},
+		Ext: map[string]interface{}{
+			"pol":          "spec_review_policy_v2",
+			"pol_decision": "approved",
+		},
 	}
 	ectA, err := ect.Create(payloadA, keyA, ect.CreateOptions{KeyID: kidA})
 	if err != nil {
@@ -69,26 +71,28 @@ func main() {
 	}
 	fmt.Println("Agent B verified root ECT and appended to ledger")
 
-	// 3) Agent B creates child ECT (par contains parent jti values per spec)
+	// 3) Agent B creates child ECT (pred contains predecessor jti values per spec)
 	keyB, _ := ect.GenerateKey()
 	kidB := "agent-b-key"
 	payloadB := &ect.Payload{
-		Iss:         agentB,
-		Aud:         []string{"spiffe://example.com/system/ledger"},
-		Iat:         now.Unix() + 1,
-		Exp:         now.Add(10 * time.Minute).Unix(),
-		Jti:         "550e8400-e29b-41d4-a716-446655440002",
-		Wid:         "wf-demo-001",
-		ExecAct:     "implement_module",
-		Par:         []string{"550e8400-e29b-41d4-a716-446655440001"},
-		Pol:         "coding_standards_v3",
-		PolDecision: ect.PolDecisionApproved,
+		Iss:     agentB,
+		Aud:     []string{"spiffe://example.com/system/ledger"},
+		Iat:     now.Unix() + 1,
+		Exp:     now.Add(10 * time.Minute).Unix(),
+		Jti:     "550e8400-e29b-41d4-a716-446655440002",
+		Wid:     "wf-demo-001",
+		ExecAct: "implement_module",
+		Pred:    []string{"550e8400-e29b-41d4-a716-446655440001"},
+		Ext: map[string]interface{}{
+			"pol":          "coding_standards_v3",
+			"pol_decision": "approved",
+		},
 	}
 	ectB, err := ect.Create(payloadB, keyB, ect.CreateOptions{KeyID: kidB})
 	if err != nil {
 		log.Fatal(err)
 	}
-	fmt.Println("Agent B created child ECT (jti=550e8400-...002, implement_module, par=[parent jti])")
+	fmt.Println("Agent B created child ECT (jti=550e8400-...002, implement_module, pred=[predecessor jti])")
 
 	// 4) Verify child ECT with DAG (ledger has task-001)
 	resolverB := ect.KeyResolver(func(kid string) (*ecdsa.PublicKey, error) {