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/tree/endpoint/core.rs

@@ -47,13 +47,6 @@ impl ChildRoute {
     }
 }
 
-/// Test leaf behavior implemented by the endpoint runtime.
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub enum LeafBehavior {
-    /// Mirrors the incoming payload back over the declared response hook.
-    Echo,
-}
-
 /// Static leaf metadata used for procedure dispatch and introspection.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct LeafSpec {
@@ -61,8 +54,6 @@ pub struct LeafSpec {
     pub name: String,
     /// Procedures supported by the leaf.
     pub procedures: Vec<String>,
-    /// Built-in behavior used by the lightweight test runtime.
-    pub behavior: LeafBehavior,
 }
 
 /// Where a frame entered the local endpoint.