test(insights): pin the clock in the two-service workout collapse case

The case built its sessions from the real clock: one copy a day before
now, the other 30 s later. The cross-source workout picker groups sessions
into fixed five-minute slots of the epoch, so whenever the suite ran in
the last 30 s of a slot the two copies landed in different slots and
counted twice. It had nothing to do with day boundaries or time zones: the
outcome followed the minute of the wall clock alone.

The case now pins Date to an instant well inside a slot. With the pin at
12:04:45Z or 23:59:45Z the case failed under UTC, Europe/Berlin and
Pacific/Kiritimati alike; at the committed 12:02:00Z it passes under all
three.

The production code reads no clock here (the block takes `now` from its
caller). What the flake exposed is the fixed slot grid itself: two copies
of one session that straddle a slot edge are not collapsed. That is a
change to the picker every workout surface shares, so it is left for a
change of its own.

src/lib/insights/__tests__/features.test.ts

@@ -559,6 +559,14 @@ describe("extractFeatures — v1.22 workouts block", () => {
   });
 
   it("counts a session synced by two services once", async () => {
+    // The cross-source picker groups sessions into fixed five-minute slots
+    // of the epoch (`pick-canonical-workout-rows.ts`). With a start taken
+    // from the real clock, the 30 s gap between the two copies crossed a
+    // slot edge whenever the test ran in the last 30 s of a slot, and the
+    // pair counted twice. The clock is pinned to an instant well inside a
+    // slot, so the case tests the collapse rather than the time of day.
+    vi.useFakeTimers({ toFake: ["Date"] });
+    vi.setSystemTime(Date.parse("2026-10-22T12:02:00Z"));
     const startedAt = new Date(Date.now() - 1 * DAY_MS);
     prismaMock.workout.findMany.mockResolvedValue([
       {
@@ -576,10 +584,14 @@ describe("extractFeatures — v1.22 workouts block", () => {
         totalDistanceM: 5010,
       },
     ]);
-    const f = await extractFeatures("user-1", false);
-    expect(f.workouts?.last7.count).toBe(1);
-    expect(f.workouts?.last7.totalDurationMin).toBe(30);
-    expect(f.workouts?.last7.totalDistanceKm).toBe(5);
+    try {
+      const f = await extractFeatures("user-1", false);
+      expect(f.workouts?.last7.count).toBe(1);
+      expect(f.workouts?.last7.totalDurationMin).toBe(30);
+      expect(f.workouts?.last7.totalDistanceKm).toBe(5);
+    } finally {
+      vi.useRealTimers();
+    }
   });
 });