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/ect/errors.go

@@ -15,15 +15,13 @@ var (
 	ErrExpired             = errors.New("ect: token expired")
 	ErrIATTooOld           = errors.New("ect: iat too far in the past")
 	ErrIATInFuture         = errors.New("ect: iat in the future")
-	ErrMissingClaims       = errors.New("ect: missing required claims (jti, exec_act, par)")
-	ErrPolPolDecisionPair  = errors.New("ect: pol and pol_decision must both be present when either is set")
-	ErrInvalidPolDecision  = errors.New("ect: invalid pol_decision value")
+	ErrMissingClaims       = errors.New("ect: missing required claims (jti, exec_act, pred)")
 	ErrReplay              = errors.New("ect: jti already seen (replay)")
 	ErrResolveKeyRequired  = errors.New("ect: ResolveKey required")
 	ErrExtSize             = errors.New("ect: ext exceeds max size (4096 bytes)")
 	ErrExtDepth            = errors.New("ect: ext exceeds max nesting depth (5)")
 	ErrInvalidJTI          = errors.New("ect: jti must be UUID format")
 	ErrInvalidWID          = errors.New("ect: wid must be UUID format when set")
-	ErrParLength           = errors.New("ect: par exceeds max length")
+	ErrPredLength          = errors.New("ect: pred exceeds max length")
 	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
 )