fix: drill-down uses route-based fetch instead of pre-loaded layout

When drilled into a sub-route, the pre-fetched diagramLayout (loaded by
content hash for the root execution) doesn't contain the sub-route's
diagram. Only use the pre-loaded layout for the root route; fall back to
useDiagramByRoute for drilled-down sub-routes.

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

ui/src/components/ProcessDiagram/ProcessDiagram.tsx

@@ -69,8 +69,12 @@ export function ProcessDiagram({
 
   const currentRouteId = routeStack[routeStack.length - 1];
 
+  // Only use the pre-fetched diagramLayout for the root route.
+  // When drilled down into a sub-route, fetch the sub-route's diagram by route ID.
+  const effectiveLayout = currentRouteId === routeId ? diagramLayout : undefined;
+
   const { sections, totalWidth, totalHeight, isLoading, error } = useDiagramData(
-    application, currentRouteId, direction, diagramLayout,
+    application, currentRouteId, direction, effectiveLayout,
   );
 
   // Collect ENDPOINT node IDs — these are always "traversed" when overlay is active