Fix WU historical current observation merge

frontend/components/dashboard/scan-terminal/__tests__/temperatureDefaultVisibilityPolicy.test.ts

@@ -5,6 +5,7 @@ import {
   __getLiveObservationLabelsForTest,
   __getObservationDisplayMetricsForTest,
   __getPeakGlowStateForTest,
+  __getWundergroundDailyHighForTest,
   __getVisibleTemperatureSeriesForTest,
   __isTemperatureSeriesVisibleByDefaultForTest,
   __mergePatchIntoHourlyForTest,
@@ -102,6 +103,22 @@ export function runTests() {
     "morning observations near the intraday observed high should not trigger peak glow before the forecast hot window",
   );
 
+  assert(
+    __getWundergroundDailyHighForTest({
+      wundergroundCurrent: { max_so_far: 26 },
+      airportCurrent: { max_so_far: 27 },
+      airportPrimary: { max_so_far: 27 },
+    } as any) === 26,
+    "WU display should use Wunderground historical.json high before airport/METAR highs",
+  );
+  assert(
+    __getWundergroundDailyHighForTest({
+      airportCurrent: { max_so_far: 27 },
+      airportPrimary: { max_so_far: 27 },
+    } as any) === null,
+    "WU display should not fall back to airport/METAR highs when historical.json is missing",
+  );
+
   const guangzhou = {
     city: "guangzhou",
     local_date: "2026-05-25",