Stability checkpoint v0.7.3 — Prometheus histograms for latency SLOs (ADR-0043). 4 histogram families (check + jwt_verify + jwks_fetch + principals_fetch) with custom-tuned buckets matching the 50ms p99 hook budget (ADR-0002). Cardinality bounded ~80 series total. 240 tests, 91.28% coverage. See ADR-0043 for SLO alert recipe.