fix: add HIERARCHY_HANDLING to handler section ELK roots

Handler section ELK roots were missing INCLUDE_CHILDREN, causing
edges between a handler compound and its children to fail with
UnsupportedGraphException (cross-hierarchy edge resolution).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/diagram/ElkDiagramRenderer.java

@@ -242,6 +242,7 @@ public class ElkDiagramRenderer implements DiagramRenderer {
             handlerRoot.setProperty(CoreOptions.DIRECTION, rootDirection);
             handlerRoot.setProperty(CoreOptions.SPACING_NODE_NODE, NODE_SPACING * 0.5);
             handlerRoot.setProperty(CoreOptions.SPACING_EDGE_NODE, EDGE_SPACING * 0.5);
+            handlerRoot.setProperty(CoreOptions.HIERARCHY_HANDLING, HierarchyHandling.INCLUDE_CHILDREN);
             handlerRoot.setProperty(org.eclipse.elk.alg.layered.options.LayeredOptions.NODE_PLACEMENT_STRATEGY,
                     NodePlacementStrategy.LINEAR_SEGMENTS);