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/python/demo.py

@@ -11,7 +11,6 @@ from ect import (
     verify,
     VerifyOptions,
     MemoryLedger,
-    POL_DECISION_APPROVED,
 )
 
 def main():
@@ -33,9 +32,11 @@ def main():
         jti=root_jti,
         wid="wf-demo-001",
         exec_act="review_requirements_spec",
-        par=[],
-        pol="spec_review_policy_v2",
-        pol_decision=POL_DECISION_APPROVED,
+        pred=[],
+        ext={
+            "pol": "spec_review_policy_v2",
+            "pol_decision": "approved",
+        },
     )
     ect_a = create(payload_a, key_a, CreateOptions(key_id=kid_a))
     print("Agent A created root ECT (jti=550e8400-..., review_requirements_spec)")
@@ -56,7 +57,7 @@ def main():
     ledger.append(ect_a, parsed.payload)
     print("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)
     key_b = generate_key()
     kid_b = "agent-b-key"
     child_jti = "550e8400-e29b-41d4-a716-446655440002"
@@ -68,12 +69,14 @@ def main():
         jti=child_jti,
         wid="wf-demo-001",
         exec_act="implement_module",
-        par=[root_jti],
-        pol="coding_standards_v3",
-        pol_decision=POL_DECISION_APPROVED,
+        pred=[root_jti],
+        ext={
+            "pol": "coding_standards_v3",
+            "pol_decision": "approved",
+        },
     )
     ect_b = create(payload_b, key_b, CreateOptions(key_id=kid_b))
-    print("Agent B created child ECT (jti=550e8400-...002, implement_module, par=[parent jti])")
+    print("Agent B created child ECT (jti=550e8400-...002, implement_module, pred=[predecessor jti])")
 
     # 4) Verify child ECT with DAG
     def resolver_b(kid):