topology: drop manager-root special case + notify three agents on reparent (#743)

hive-c0re/src/server.rs

@@ -187,13 +187,15 @@ async fn dispatch(req: &HostRequest, coord: Arc<Coordinator>) -> HostResponse {
             }
             HostRequest::SetParent { child, new_parent } => {
                 tracing::info!(%child, ?new_parent, "set_parent");
-                crate::topology::set_parent(child, new_parent.as_deref())
+                // `reparent_with_notify` wraps `topology::set_parent`
+                // with the three notification messages (#743) + the
+                // ContainerView rescan. Idempotent same-parent calls
+                // skip both the messages and the disk write per the
+                // topology fast-path.
+                coord
+                    .reparent_with_notify(child, new_parent.as_deref())
+                    .await
                     .map_err(anyhow::Error::msg)?;
-                // ContainerView.parent is read from topology.json — a
-                // change here means every container row potentially
-                // moves in the dashboard tree. Rescan + diff-emit so
-                // open viewers repaint without polling.
-                coord.rescan_containers_and_emit().await;
                 HostResponse::success()
             }
         })