fix: use graph root + edge walk to separate main flow from handlers

Root cause: graph.getNodes() is a flat list with duplicates — handler
compound children appear both nested inside their parent AND as
top-level entries. The previous separation tried to filter the flat
list but missed the duplicates, leaving handler children in rootNode.

New approach: walk from graph.getRoot() following non-ERROR edges to
discover main flow nodes. Edges targeting handler compounds (ON_EXCEPTION,
ON_COMPLETION) are not followed. This cleanly separates main flow from
handler sections using the graph's own structure.

Falls back to flat list filtering (old behavior) when graph.getRoot()
is null (legacy/test graphs).

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

@@ -206,24 +206,59 @@ public class ElkDiagramRenderer implements DiagramRenderer {
         Map<String, Color> nodeColors = new HashMap<>();
         Set<String> compoundNodeIds = new HashSet<>();
 
-        // Separate handler sections from main flow nodes.
-        // graph.getNodes() is a FLAT list that includes handler compound children
-        // as top-level entries. We must identify and exclude them.
+        // Build a lookup from the flat nodes list (graph.getNodes() contains
+        // duplicates — children appear both nested AND top-level). Use this for
+        // resolving edge endpoints to RouteNode objects.
+        Map<String, RouteNode> nodeById = new HashMap<>();
+        if (graph.getNodes() != null) {
+            for (RouteNode rn : graph.getNodes()) {
+                nodeById.put(rn.getId(), rn);
+            }
+        }
+
+        // Separate main flow from handler sections using graph structure:
+        // - Start from graph.getRoot() and walk FLOW edges for main flow
+        // - Handler sections are top-level nodes of HANDLER_SECTION_TYPES
+        // This avoids the problem of the flat nodes list containing duplicates.
+        Set<String> mainNodeIds = new HashSet<>();
         List<RouteNode> mainNodes = new ArrayList<>();
         List<RouteNode> handlerNodes = new ArrayList<>();
-        Set<String> handlerDescendantIds = new HashSet<>();
+
+        // Collect main flow node IDs by walking from root along non-ERROR edges.
+        // graph.getRoot() provides the entry point; ERROR edges lead to handler sections.
+        if (graph.getRoot() != null && graph.getEdges() != null) {
+            mainNodeIds.add(graph.getRoot().getId());
+            boolean changed = true;
+            while (changed) {
+                changed = false;
+                for (RouteEdge re : graph.getEdges()) {
+                    if (re.getEdgeType() == RouteEdge.EdgeType.ERROR) continue;
+                    if (mainNodeIds.contains(re.getSource()) && !mainNodeIds.contains(re.getTarget())) {
+                        // Don't follow edges INTO handler compounds
+                        RouteNode target = nodeById.get(re.getTarget());
+                        if (target != null && target.getType() != null
+                                && HANDLER_SECTION_TYPES.contains(target.getType())) {
+                            continue;
+                        }
+                        mainNodeIds.add(re.getTarget());
+                        changed = true;
+                    }
+                }
+            }
+        }
+
+        // Build node lists — deduplicate (flat list has duplicates)
+        Set<String> seen = new HashSet<>();
         if (graph.getNodes() != null) {
-            // First pass: identify handler compounds and collect their descendant IDs
             for (RouteNode rn : graph.getNodes()) {
+                if (seen.contains(rn.getId())) continue;
+                seen.add(rn.getId());
                 if (rn.getType() != null && HANDLER_SECTION_TYPES.contains(rn.getType())
                         && rn.getChildren() != null && !rn.getChildren().isEmpty()) {
                     handlerNodes.add(rn);
-                    collectDescendantIds(rn.getChildren(), handlerDescendantIds);
-                }
-            }
-            // Second pass: main flow = everything that isn't a handler or handler descendant
-            for (RouteNode rn : graph.getNodes()) {
-                if (!handlerNodes.contains(rn) && !handlerDescendantIds.contains(rn.getId())) {
+                } else if (mainNodeIds.isEmpty() || mainNodeIds.contains(rn.getId())) {
+                    // When no root is available (legacy/test graphs), include all
+                    // non-handler nodes as main flow (fallback to old behavior)
                     mainNodes.add(rn);
                 }
             }