fix(reasoning): preserve investigation ownership across selection and question rejection

lib/graph/utils.js

@@ -249,7 +249,7 @@ function semanticSignature(candidate) {
   );
 }
 
-function classifyCandidateOrdering(candidates) {
+function classifyCandidateOrdering(candidates, activeNodeId = null) {
   const displayOrder = buildCandidateDisplayOrder(candidates);
   const best = displayOrder[0] ?? null;
   if (!best) {
@@ -319,15 +319,21 @@ function classifyCandidateOrdering(candidates) {
   }
 
   if (topStructuralCandidates.length > 0) {
+    const activeTiedCandidate = activeNodeId
+      ? topStructuralCandidates.find((candidate) => candidate.nodeId === activeNodeId)
+      : null;
+
     return {
       displayOrder,
-      best,
+      best: activeTiedCandidate || best,
       leadingCandidates: topStructuralCandidates,
       status: "selected",
       tieType: "complete_unresolved_tie",
-      usedAlphabeticalOrdering: true,
+      usedAlphabeticalOrdering: activeTiedCandidate ? false : true,
       reason:
-        "Leading candidates remained tied after score, structural, and semantic checks, so the stable deterministic display order was used as the final fallback.",
+        activeTiedCandidate
+          ? "Leading candidates remained tied after score, structural, and semantic checks, so existing active investigation ownership was preserved."
+          : "Leading candidates remained tied after score, structural, and semantic checks, so the stable deterministic display order was used as the final fallback.",
     };
   }
 
@@ -624,6 +630,7 @@ export function selectActiveUnknownCandidate(graph, resolvedNodeIds) {
       ...candidate,
       node,
     })),
+    graph?.activeUnknownNodeId ?? null,
   );
 
   if (selection.status === "ambiguous") {
@@ -690,7 +697,10 @@ export function explainUnknownSelection(graph, resolvedNodeIds = []) {
     ...scoreUnknownCandidate(graph, node, resolvedNodeIds),
   }));
 
-  const selection = classifyCandidateOrdering(candidates);
+  const selection = classifyCandidateOrdering(
+    candidates,
+    graph?.activeUnknownNodeId ?? null,
+  );
   const orderedCandidates = selection.displayOrder;
   const selected = selection.best;
   const competitors = orderedCandidates