feat: add ExecutionDiagram wrapper component

Composes ProcessDiagram with execution overlay data, exchange summary
bar, resizable splitter, and detail panel into a single root component.

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

ui/src/components/ExecutionDiagram/index.ts

@@ -0,0 +1,2 @@
+export { ExecutionDiagram } from './ExecutionDiagram';
+export type { NodeExecutionState, IterationInfo, DetailTab } from './types';