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.

treetest/src/sim/runtime/events/application.rs

@@ -7,7 +7,7 @@
 
 use unshell::protocol::{CallMessage, PacketHeader};
 
-use crate::model::{EndpointProcedureKind, EndpointProcedureSpec, NodeId};
+use crate::model::{EndpointProcedureKind, EndpointProcedureSpec, LeafKind, NodeId};
 
 use super::super::super::types::{SimError, Simulation};
 
@@ -17,13 +17,32 @@ impl Simulation {
     pub(super) fn handle_application_call(
         &mut self,
         node_id: NodeId,
-        _header: &PacketHeader,
+        header: &PacketHeader,
         message: &CallMessage,
     ) -> Result<(), SimError> {
         let Some(hook) = &message.response_hook else {
             return Ok(());
         };
 
+        if let Some(leaf_name) = &header.dst_leaf {
+            let leaf = self.require_leaf(node_id, leaf_name)?.clone();
+            match leaf.kind {
+                LeafKind::Echo => {
+                    let frame = self.make_endpoint_data_frame(
+                        node_id,
+                        hook.return_path.clone(),
+                        hook.hook_id,
+                        message.procedure_id.clone(),
+                        message.data.clone(),
+                        true,
+                    )?;
+                    self.record_trace(node_id, format!("leaf {leaf_name} echoed {} bytes", message.data.len()));
+                    self.process_local_frame(node_id, frame)?;
+                }
+            }
+            return Ok(());
+        }
+
         // Clone the procedure spec once so later reply generation can borrow the
         // rest of the simulator state freely.
         let procedure = self