Add MSW infrastructure: handlers, fixtures, request-log, and Vitest setup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mocks/handlers/compare.js

@@ -0,0 +1,14 @@
+/**
+ * MSW handlers for comparison dashboard endpoints.
+ * @module mocks/handlers/compare
+ */
+import { http, HttpResponse } from "msw";
+import compareData from "../fixtures/compare.json";
+import { captureRequest } from "../request-log.js";
+
+export const compareHandlers = [
+  http.get("/api/compare", ({ request }) => {
+    captureRequest("GET", "/api/compare");
+    return HttpResponse.json(compareData);
+  }),
+];