//! Observability hooks (`ARCHITECTURE.md` §6.4).
//!
//! `tracing` spans on gateway events, Prometheus metrics on
//! counts/latencies/queue depths, optional OTLP export. **None of this is on the
//! wire** — it is local instrumentation, opt-in, living here because it sits in
//! the extension layer. Implemented in Phase 4 (§11.4).
