fix: proper LCA and bounding box for multi-root ELK layout

1. findCommonParent: replaced with correct lowest common ancestor
   algorithm using ancestor set intersection (previous version only
   walked from node 'a', not a true LCA)

2. Bounding box: compute totalWidth/totalHeight from actual positioned
   node coordinates instead of rootNode.getWidth/Height. The rootNode
   dimensions don't account for handler sections in separate ELK roots.

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

@@ -355,8 +355,25 @@ public class ElkDiagramRenderer implements DiagramRenderer {
             positionedEdges.add(new PositionedEdge(sourceId, targetId, label, points));
         }
 
-        double totalWidth = rootNode.getWidth();
-        double totalHeight = rootNode.getHeight();
+        // Compute bounding box from actual positioned node coordinates
+        // (not rootNode dimensions, which ignore handler roots)
+        double totalWidth = 0;
+        double totalHeight = 0;
+        for (PositionedNode pn : positionedNodes) {
+            double right = pn.x() + pn.width();
+            double bottom = pn.y() + pn.height();
+            if (right > totalWidth) totalWidth = right;
+            if (bottom > totalHeight) totalHeight = bottom;
+            // Also check children bounds for compounds
+            if (pn.children() != null) {
+                for (PositionedNode child : pn.children()) {
+                    double cr = pn.x() + child.x() + child.width();
+                    double cb = pn.y() + child.y() + child.height();
+                    if (cr > totalWidth) totalWidth = cr;
+                    if (cb > totalHeight) totalHeight = cb;
+                }
+            }
+        }
 
         DiagramLayout layout = new DiagramLayout(totalWidth, totalHeight, positionedNodes, positionedEdges);
         return new LayoutResult(layout, nodeColors, compoundInfos);
@@ -572,19 +589,25 @@ public class ElkDiagramRenderer implements DiagramRenderer {
         return current;
     }
 
+    /** Proper lowest common ancestor of two ELK nodes. */
     private ElkNode findCommonParent(ElkNode a, ElkNode b) {
-        if (a.getParent() == b.getParent()) {
-            return a.getParent();
+        // Collect all ancestors of 'a' (including a itself)
+        Set<ElkNode> ancestorsOfA = new HashSet<>();
+        ElkNode current = a;
+        while (current != null) {
+            ancestorsOfA.add(current);
+            current = current.getParent();
         }
-        // If one is the parent of the other
-        if (a.getParent() != null && a.getParent() == b) return b;
-        if (b.getParent() != null && b.getParent() == a) return a;
-        // Default: root (grandparent)
-        ElkNode parent = a.getParent();
-        while (parent != null && parent.getParent() != null) {
-            parent = parent.getParent();
+        // Walk up from 'b' until we find a common ancestor
+        current = b;
+        while (current != null) {
+            if (ancestorsOfA.contains(current)) {
+                return current;
+            }
+            current = current.getParent();
         }
-        return parent != null ? parent : a.getParent();
+        // Fallback: root of 'a'
+        return getElkRoot(a);
     }
 
     private double getAbsoluteX(ElkNode node, ElkNode root) {