perf(search): replace repeated result scans with indexed lookups

frontend/src/components/SearchLayout.test.tsx

@@ -253,4 +253,64 @@ describe("SearchLayout product events", () => {
     expect(resultLookupCount).toBe(lookupCountAfterResults);
     findSpy.mockRestore();
   });
-});
+
+  it("keeps the first search result authoritative when duplicate ids arrive", async () => {
+    vi.stubGlobal("fetch", vi.fn((input: RequestInfo | URL) => {
+      const url = String(input);
+      if (url.endsWith("/api/search")) {
+        return Promise.resolve(jsonResponse({
+          results: [
+            {
+              id: 303,
+              source_message_id: "<first-source@example.com>",
+              subject: "첫 번째 중복 ID 결과",
+              sender: "first@example.com",
+              date: "2026-05-20T09:00:00Z",
+              snippet: "첫 번째 검색 근거",
+              thread_id: "thread-first",
+              reply_count: 2,
+              score: 0.91,
+            },
+            {
+              id: 303,
+              source_message_id: "<second-source@example.com>",
+              subject: "두 번째 중복 ID 결과",
+              sender: "second@example.com",
+              date: "2026-05-20T09:01:00Z",
+              snippet: "두 번째 검색 근거",
+              thread_id: "thread-second",
+              reply_count: 3,
+              score: 0.82,
+            },
+          ],
+        }));
+      }
+      if (url.endsWith("/api/search/answer")) {
+        return Promise.resolve(jsonResponse({ answer: null, citations: [] }));
+      }
+      if (url.includes("/api/ontology/relationships?")) {
+        return Promise.resolve(jsonResponse([]));
+      }
+      throw new Error(`Unexpected fetch: ${url}`);
+    }));
+
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = createRoot(container);
+    await act(async () => {
+      root?.render(<SearchLayout />);
+    });
+
+    await waitForCondition(() =>
+      container?.querySelector("section[aria-label='맥락 검색 결과 상세'] h2") !== null,
+    );
+    expect(
+      container.querySelector("section[aria-label='맥락 검색 결과 상세'] h2")?.textContent,
+    ).toBe("첫 번째 중복 ID 결과");
+    expect(getRecordedProductEvents().some((event) =>
+      event.name === "context_search_result_opened" &&
+      event.payload.result_id === 303 &&
+      event.payload.rank_bucket === "top_1",
+    )).toBe(true);
+  });
+});

frontend/src/components/SearchLayout.tsx

@@ -462,12 +462,19 @@ export function SearchLayout() {
     return results;
   }, [activeFilter, results]);
 
-  // ⚡ Bolt: Memoize activeResult to prevent O(N) re-renders
-  const activeResult = useMemo(() => (
-    filteredResults.find((result) => result.id === activeResultId) ??
+  const resultById = useMemo(() => {
+    const map = new Map<number, { result: SearchResultItem; index: number }>();
+    filteredResults.forEach((result, index) => {
+      // Preserve the first-match semantics of the previous find/findIndex path.
+      if (!map.has(result.id)) map.set(result.id, { result, index });
+    });
+    return map;
+  }, [filteredResults]);
+
+  const activeResult =
+    (activeResultId !== null ? resultById.get(activeResultId)?.result : null) ??
     filteredResults[0] ??
-    null
-  ), [filteredResults, activeResultId]);
+    null;
   const activeOntologySourceKey = ontologySourceKey(activeResult);
   const activeOntologyUrl = activeResult
     ? buildOntologyUrl(activeResult)
@@ -496,7 +503,7 @@ export function SearchLayout() {
   useEffect(() => {
     if (!activeResult || loading) return;
 
-    const resultIndex = filteredResults.findIndex((result) => result.id === activeResult.id);
+    const resultIndex = resultById.get(activeResult.id)?.index ?? -1;
     const eventKey = `${searchSessionIdRef.current}:${activeResult.id}`;
     if (lastOpenedResultKeyRef.current === eventKey) return;
     lastOpenedResultKeyRef.current = eventKey;
@@ -509,7 +516,7 @@ export function SearchLayout() {
       rank_bucket: bucketSearchRank(resultIndex < 0 ? 0 : resultIndex),
       confidence: activeConfidence,
     });
-  }, [activeConfidence, activeResult, filteredResults, loading]);
+  }, [activeConfidence, activeResult, resultById, loading]);
 
   useEffect(() => {
     if (!activeOntologyUrl || !activeOntologySourceKey) return;