- ported from ChironeWp3 (22 DecisionType, append-only jsonl, monotonic seq)
- added decision_retracted type + retracts field for step-level rollback (D15):
the retracted decision stays in the audit log, effective_decisions() (Task A5)
will exclude it from the active view
- 5 tests: retract marker + monotonic seq + retracts default + empty session +
literal includes retracted. All 13 harness tests pass.