feat: latency heatmap overlay on process diagram (#94)

Add latencyHeatmap prop to ProcessDiagram that colors nodes green→yellow→red
based on their relative contribution to route latency (pctOfRoute). Shows avg
duration label on each node. Threaded through CompoundNode for nested EIP
patterns. Heatmap is active only when no execution overlay is present.

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

ui/src/components/ProcessDiagram/CompoundNode.tsx

@@ -1,5 +1,5 @@
 import type { DiagramNode as DiagramNodeType, DiagramEdge as DiagramEdgeType } from '../../api/queries/diagrams';
-import type { NodeConfig } from './types';
+import type { NodeConfig, LatencyHeatmapEntry } from './types';
 import type { NodeExecutionState, IterationInfo } from '../ExecutionDiagram/types';
 import { colorForType, isCompoundType, iconForType, type IconElement } from './node-colors';
 import { DiagramNode } from './DiagramNode';
@@ -27,6 +27,7 @@ interface CompoundNodeProps {
   iterationState?: Map<string, IterationInfo>;
   /** Called when user changes iteration on a compound stepper */
   onIterationChange?: (compoundNodeId: string, iterationIndex: number) => void;
+  latencyHeatmap?: Map<string, LatencyHeatmapEntry>;
   onNodeClick: (nodeId: string) => void;
   onNodeDoubleClick?: (nodeId: string) => void;
   onNodeEnter: (nodeId: string) => void;
@@ -36,7 +37,7 @@ interface CompoundNodeProps {
 export function CompoundNode({
   node, edges, parentX = 0, parentY = 0,
   selectedNodeId, hoveredNodeId, nodeConfigs, executionOverlay,
-  overlayActive, iterationState, onIterationChange,
+  overlayActive, iterationState, onIterationChange, latencyHeatmap,
   onNodeClick, onNodeDoubleClick, onNodeEnter, onNodeLeave,
 }: CompoundNodeProps) {
   const x = (node.x ?? 0) - parentX;
@@ -61,7 +62,7 @@ export function CompoundNode({
 
   const childProps = {
     edges, selectedNodeId, hoveredNodeId, nodeConfigs, executionOverlay,
-    overlayActive, iterationState, onIterationChange,
+    overlayActive, iterationState, onIterationChange, latencyHeatmap,
     onNodeClick, onNodeDoubleClick, onNodeEnter, onNodeLeave,
   };
 
@@ -243,6 +244,7 @@ function renderChildren(
             config={child.id ? props.nodeConfigs?.get(child.id) : undefined}
             executionState={props.executionOverlay?.get(child.id ?? '')}
             overlayActive={props.overlayActive}
+            heatmapEntry={child.id ? props.latencyHeatmap?.get(child.id) : undefined}
             onClick={() => child.id && props.onNodeClick(child.id)}
             onDoubleClick={() => child.id && props.onNodeDoubleClick?.(child.id)}
             onMouseEnter={() => child.id && props.onNodeEnter(child.id)}