Phase 40: Routing Engine + Policy

- RoutingEngine with RouteDecision (model_pattern → provider)
- config/routing.toml: rules, fallback chain, cost gating
- Per-provider Usage tracking in /v1/usage response
- 12 gateway tests green

crates/aibridge/src/lib.rs

@@ -3,5 +3,6 @@ pub mod context;
 pub mod continuation;
 pub mod provider;
 pub mod providers;
+pub mod routing;
 pub mod service;
 pub mod tree_split;