Adds comprehensive test coverage for the run-audit system across the engine package, including lane session audit tests (triage, executor, reviewer, merger, heartbeat) and runtime audit invariants, plus a backcompat test for no-auditor scenarios.
Fusion-Task-Id: FN-5556
Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5556
