Observability & cost governance (Stage 18, final stage of the 18-week plan): boundary-captured metrics, multi-dimensional budgets, explainable model routing, evaluation regression, version provenance. Metrics live at the decorator boundary, not in the loop path (blueprint D2); one run, three projections - Trajectory (training, Stage 14) / AuditEvent (governance, Stage 9) / Metrics (operations, Stage 18) share the same boundaries (blueprint D1).