feat: render EIP_CIRCUIT_BREAKER as compound container with main/fallback lanes

Follow the DO_TRY pattern: virtual _CB_MAIN wrapper for main path children,
onFallback rendered as _CB_FALLBACK section with purple dashed border.

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

ui/src/components/ProcessDiagram/CompoundNode.tsx

@@ -65,8 +65,8 @@ export function CompoundNode({
     onNodeClick, onNodeDoubleClick, onNodeEnter, onNodeLeave,
   };
 
-  // _TRY_BODY: transparent wrapper — no header, no border, just layout
-  if (node.type === '_TRY_BODY') {
+  // _TRY_BODY / _CB_MAIN: transparent wrapper — no header, no border, just layout
+  if (node.type === '_TRY_BODY' || node.type === '_CB_MAIN') {
     return (
       <g transform={`translate(${x}, ${y})`}>
         {renderInternalEdges(internalEdges, absX, absY, executionOverlay)}
@@ -75,6 +75,24 @@ export function CompoundNode({
     );
   }
 
+  // _CB_FALLBACK: section styling with EIP purple
+  if (node.type === '_CB_FALLBACK') {
+    const fallbackColor = '#7C3AED'; // EIP purple
+    return (
+      <g data-node-id={node.id} transform={`translate(${x}, ${y})`}>
+        <rect x={0} y={0} width={w} height={h} rx={CORNER_RADIUS}
+          fill={fallbackColor} fillOpacity={0.06} />
+        <rect x={0} y={0} width={w} height={h} rx={CORNER_RADIUS}
+          fill="none" stroke={fallbackColor} strokeWidth={1} strokeOpacity={0.4} strokeDasharray="4 2" />
+        <text x={8} y={12} fill={fallbackColor} fontSize={10} fontWeight={600}>
+          fallback
+        </text>
+        {renderInternalEdges(internalEdges, absX, absY, executionOverlay)}
+        {renderChildren(node, absX, absY, childProps)}
+      </g>
+    );
+  }
+
   // DO_CATCH / DO_FINALLY: section-like styling (tinted bg, thin border, label)
   if (node.type === 'DO_CATCH' || node.type === 'DO_FINALLY') {
     const sectionLabel = node.type === 'DO_CATCH'