feat: add iteration stepper to compound nodes and thread overlay props

Add a left/right stepper widget to compound node headers (LOOP, SPLIT,
MULTICAST) when iteration overlay data is present. Thread executionOverlay,
overlayActive, iterationState, and onIterationChange props through
ProcessDiagram -> CompoundNode -> children and ProcessDiagram ->
ErrorSection -> children so leaf DiagramNode instances render with
execution state (green/red badges, dimming for skipped nodes).

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

ui/src/components/ProcessDiagram/ProcessDiagram.tsx

@@ -54,7 +54,10 @@ export function ProcessDiagram({
   className,
   diagramLayout,
   executionOverlay,
+  iterationState,
+  onIterationChange,
 }: ProcessDiagramProps) {
+  const overlayActive = !!executionOverlay;
   // Route stack for drill-down navigation
   const [routeStack, setRouteStack] = useState<string[]>([routeId]);
 
@@ -248,6 +251,9 @@ export function ProcessDiagram({
                     hoveredNodeId={toolbar.hoveredNodeId}
                     nodeConfigs={nodeConfigs}
                     executionOverlay={executionOverlay}
+                    overlayActive={overlayActive}
+                    iterationState={iterationState}
+                    onIterationChange={onIterationChange}
                     onNodeClick={handleNodeClick}
                     onNodeDoubleClick={handleNodeDoubleClick}
                     onNodeEnter={toolbar.onNodeEnter}
@@ -262,6 +268,8 @@ export function ProcessDiagram({
                   isHovered={toolbar.hoveredNodeId === node.id}
                   isSelected={selectedNodeId === node.id}
                   config={node.id ? nodeConfigs?.get(node.id) : undefined}
+                  executionState={executionOverlay?.get(node.id ?? '')}
+                  overlayActive={overlayActive}
                   onClick={() => node.id && handleNodeClick(node.id)}
                   onDoubleClick={() => node.id && handleNodeDoubleClick(node.id)}
                   onMouseEnter={() => node.id && toolbar.onNodeEnter(node.id)}
@@ -283,6 +291,9 @@ export function ProcessDiagram({
               hoveredNodeId={toolbar.hoveredNodeId}
               nodeConfigs={nodeConfigs}
               executionOverlay={executionOverlay}
+              overlayActive={overlayActive}
+              iterationState={iterationState}
+              onIterationChange={onIterationChange}
               onNodeClick={handleNodeClick}
               onNodeDoubleClick={handleNodeDoubleClick}
               onNodeEnter={toolbar.onNodeEnter}