test(insights): date the comparison's sleep readings by calendar day

The readings sat at now minus n times 24 hours, which keeps the wall-clock
hour only between two clock changes. Run late in the evening after the
October change, two of them fell into the same night and the average read
525 minutes instead of 420.

src/lib/insights/__tests__/comparison-snapshot-window.test.ts

@@ -305,10 +305,21 @@ describe("buildComparisonSnapshotForUser — output-equivalence to the raw-avera
           ? { _avg: { value: args.where.measuredAt.lte ? 82 : 80 } }
           : { _avg: { value: null } },
     );
-    // Seven-hour nights, stored in minutes.
+    // Seven-hour nights, stored in minutes. Each at midday UTC of its own
+    // calendar day: `now - n * DAY` keeps the wall-clock hour only between
+    // two clock changes, and run late in the evening across one, two of the
+    // readings fell into the same night.
+    const today = new Date();
     measurementFindMany.mockResolvedValue(
       Array.from({ length: 5 }, (_, i) => ({
-        measuredAt: new Date(Date.now() - (i + 2) * DAY),
+        measuredAt: new Date(
+          Date.UTC(
+            today.getUTCFullYear(),
+            today.getUTCMonth(),
+            today.getUTCDate() - (i + 2),
+            12,
+          ),
+        ),
         value: 420,
         sleepStage: null,
         source: "APPLE_HEALTH",