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/ledger.go

@@ -12,7 +12,7 @@ type LedgerEntry struct {
 	TaskID               string    `json:"task_id"`
 	AgentID              string    `json:"agent_id"`
 	Action               string    `json:"action"`
-	Parents              []string  `json:"parents"`
+	Predecessors         []string  `json:"predecessors"`
 	ECTJWS               string    `json:"ect_jws"`
 	SignatureVerified    bool      `json:"signature_verified"`
 	VerificationTime     time.Time `json:"verification_timestamp"`
@@ -66,7 +66,7 @@ func (m *MemoryLedger) Append(ectJWS string, payload *Payload) (int64, error) {
 		TaskID:            payload.Jti, // task id = jti per spec
 		AgentID:           payload.Iss,
 		Action:            payload.ExecAct,
-		Parents:           append([]string(nil), payload.Par...),
+		Predecessors:      append([]string(nil), payload.Pred...),
 		ECTJWS:            ectJWS,
 		SignatureVerified: true,
 		VerificationTime:  time.Now().UTC(),