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/tests/test_create.py

@@ -13,7 +13,6 @@ from ect import (
     CreateOptions,
     verify,
     VerifyOptions,
-    POL_DECISION_APPROVED,
 )
 
 
@@ -27,9 +26,7 @@ def test_create_roundtrip():
         exp=now + 600,
         jti="e4f5a6b7-c8d9-0123-ef01-234567890abc",
         exec_act="review_spec",
-        par=[],
-        pol="spec_review_policy_v2",
-        pol_decision=POL_DECISION_APPROVED,
+        pred=[],
     )
     compact = create(payload, key, CreateOptions(key_id="agent-a-key-1"))
     assert compact