refactor(workspace): enforce layered crate boundaries

crates/aether-gateway/frontdoor/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "aether-gateway-frontdoor"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "HTTP frontdoor middleware and request lifecycle primitives for Aether gateway"
+
+[dependencies]
+aether-ai-formats.workspace = true
+axum.workspace = true
+bytes.workspace = true
+http.workspace = true
+tokio.workspace = true
+tracing.workspace = true
+uuid.workspace = true
+
+[dev-dependencies]
+futures-util.workspace = true
+serde_json.workspace = true
+tower = { version = "0.5", features = ["util"] }
+tracing-subscriber.workspace = true