Align protocol runtime with spec boundaries

Move demo leaf echo behavior out of the core protocol runtime, treat procedure IDs as opaque protocol fields, and return direct registered children in endpoint introspection to match the spec.

src/protocol/introspection.rs

@@ -9,6 +9,8 @@ pub const INTROSPECTION_PROCEDURE_ID: &str = "";
 /// Endpoint-wide introspection payload.
 #[derive(Archive, Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
 pub struct EndpointIntrospection {
+    /// Direct child path segments currently registered under this endpoint.
+    pub sub_endpoints: Vec<String>,
     /// Hosted leaves and their supported procedures.
     pub leaves: Vec<LeafIntrospectionSummary>,
 }