fix(workbench): stabilize investigation presentation rendering

Keep investigation graphs stable during updates, defer expensive chart rendering, and prevent stale results when switching tasks. Adopt reference annotations and consolidate frontend documentation.

src/features/workbench/components/investigation-task-report.tsx

@@ -181,10 +181,11 @@ export function InvestigationTaskReport({
   // progress snapshot owns its whole graph, including an explicit null. A prior
   // round's final report must never hide the new round's progress.
   const { currentFinalReport, previousRound, olderRevision, insufficient } = getReportPresentationState(task, snapshot);
+  const savedDiagram = task.latest_progress?.progress.diagram;
+  const cumulativeDiagram = savedDiagram?.edges != null ? savedDiagram : null;
   const diagram = live
-    ? progress && !currentFinalReport ? progress.progress.diagram : investigationDiagram(task, snapshot)
+    ? cumulativeDiagram ?? (progress && !currentFinalReport ? progress.progress.diagram : investigationDiagram(task, snapshot))
     : selection.diagram;
-  const diagramRunning = Boolean(live && !currentFinalReport && progress && task.active_run?.run_id === progress.run_id && task.active_run.status === "running");
   const latestAvailable = !live;
   const { currentInsufficient } = getReportPresentationState(task, task.latest_report);
   const pendingActions = task.actions.filter(isPendingInvestigationAction);
@@ -531,7 +532,6 @@ export function InvestigationTaskReport({
         <InvestigationDiagramView
           key={live ? `live:${task.task_id}` : `report:${snapshot?.revision}`}
           diagram={diagram}
-          running={diagramRunning}
           onLocate={onLocate}
           renderDetails={(node) => (
             <>