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/ect/__init__.py

@@ -1,13 +1,10 @@
 # WIMSE Execution Context Tokens (ECT) — Python reference implementation
-# draft-nennemann-wimse-execution-context-00
+# draft-nennemann-wimse-execution-context-01
 
 from ect.types import (
     ECT_TYPE,
-    POL_DECISION_APPROVED,
-    POL_DECISION_REJECTED,
-    POL_DECISION_PENDING_HUMAN_REVIEW,
+    ECT_TYPE_LEGACY,
     Payload,
-    valid_pol_decision,
 )
 from ect.create import create, generate_key, CreateOptions, default_create_options
 from ect.verify import (
@@ -30,11 +27,8 @@ from ect.jti_cache import JTICache, new_jti_cache
 
 __all__ = [
     "ECT_TYPE",
-    "POL_DECISION_APPROVED",
-    "POL_DECISION_REJECTED",
-    "POL_DECISION_PENDING_HUMAN_REVIEW",
+    "ECT_TYPE_LEGACY",
     "Payload",
-    "valid_pol_decision",
     "create",
     "generate_key",
     "CreateOptions",