Factor out mock date helper in tracker tests

- reuse a shared `withMockDate` helper for date-sensitive query tests
- make monthly rollup assertions key off `videoId` instead of row order

src/core/services/immersion-tracker/__tests__/query.test.ts

@@ -81,6 +81,34 @@ function cleanupDbPath(dbPath: string): void {
   }
 }
 
+function withMockDate<T>(fixedDate: Date, run: (realDate: typeof Date) => T): T {
+  const realDate = Date;
+  const fixedDateMs = fixedDate.getTime();
+
+  type MockDateArgs = [any, any, any, any, any, any, any];
+
+  class MockDate extends Date {
+    constructor(...args: MockDateArgs) {
+      if (args.length === 0) {
+        super(fixedDateMs);
+      } else {
+        super(...args);
+      }
+    }
+
+    static override now(): number {
+      return fixedDateMs;
+    }
+  }
+
+  globalThis.Date = MockDate as DateConstructor;
+  try {
+    return run(realDate);
+  } finally {
+    globalThis.Date = realDate;
+  }
+}
+
 test('getSessionSummaries returns sessionId and canonicalTitle', () => {
   const dbPath = makeDbPath();
   const db = new Database(dbPath);
@@ -790,25 +818,8 @@ test('getTrendsDashboard keeps local-midnight session buckets separate', () => {
 test('getTrendsDashboard month grouping spans every touched calendar month and keeps progress monthly', () => {
   const dbPath = makeDbPath();
   const db = new Database(dbPath);
-  const RealDate = Date;
-
-  class MockDate extends Date {
-    constructor(...args: any[]) {
-      type MockDateArgs = [any, any, any, any, any, any, any];
-      if (args.length === 0) {
-        super(new RealDate(2026, 2, 1, 12, 0, 0).getTime());
-      } else {
-        super(...(args as MockDateArgs));
-      }
-    }
-
-    static override now(): number {
-      return new RealDate(2026, 2, 1, 12, 0, 0).getTime();
-    }
-  }
-
+  withMockDate(new Date(2026, 2, 1, 12, 0, 0), (RealDate) => {
     try {
-    globalThis.Date = MockDate as DateConstructor;
       ensureSchema(db);
       const stmts = createTrackerPreparedStatements(db);
       const febVideoId = getOrCreateVideoRecord(db, 'local:/tmp/feb-trends.mkv', {
@@ -988,10 +999,10 @@ test('getTrendsDashboard month grouping spans every touched calendar month and k
       dashboard.activity.watchTime.map((point) => point.label),
     );
     } finally {
-    globalThis.Date = RealDate;
       db.close();
       cleanupDbPath(dbPath);
     }
+  });
 });
 
 test('getQueryHints reads all-time totals from lifetime summary', () => {
@@ -1067,25 +1078,9 @@ test('getQueryHints reads all-time totals from lifetime summary', () => {
 test('getQueryHints computes weekly new-word cutoff from calendar midnights', () => {
   const dbPath = makeDbPath();
   const db = new Database(dbPath);
-  const RealDate = Date;
-
-  class MockDate extends Date {
-    constructor(...args: any[]) {
-      type MockDateArgs = [any, any, any, any, any, any, any];
-      if (args.length === 0) {
-        super(new RealDate(2026, 2, 15, 12, 0, 0).getTime());
-      } else {
-        super(...(args as MockDateArgs));
-      }
-    }
-
-    static override now(): number {
-      return new RealDate(2026, 2, 15, 12, 0, 0).getTime();
-    }
-  }
 
+  withMockDate(new Date(2026, 2, 15, 12, 0, 0), (RealDate) => {
     try {
-    globalThis.Date = MockDate as DateConstructor;
       ensureSchema(db);
 
       const insertWord = db.prepare(
@@ -1129,10 +1124,10 @@ test('getQueryHints computes weekly new-word cutoff from calendar midnights', ()
       const hints = getQueryHints(db);
       assert.equal(hints.newWordsThisWeek, 1);
     } finally {
-    globalThis.Date = RealDate;
       db.close();
       cleanupDbPath(dbPath);
     }
+  });
 });
 
 test('getQueryHints counts new words by distinct headword first-seen time', () => {
@@ -1522,11 +1517,12 @@ test('getMonthlyRollups derives rate metrics from stored monthly totals', () =>
 
     const rows = getMonthlyRollups(db, 1);
     assert.equal(rows.length, 2);
-    assert.equal(rows[1]?.cardsPerHour, 30);
-    assert.equal(rows[1]?.tokensPerMin, 3);
-    assert.equal(rows[1]?.lookupHitRate ?? null, null);
-    assert.equal(rows[0]?.cardsPerHour ?? null, null);
-    assert.equal(rows[0]?.tokensPerMin ?? null, null);
+    const rowsByVideoId = new Map(rows.map((row) => [row.videoId, row]));
+    assert.equal(rowsByVideoId.get(2)?.cardsPerHour, 30);
+    assert.equal(rowsByVideoId.get(2)?.tokensPerMin, 3);
+    assert.equal(rowsByVideoId.get(2)?.lookupHitRate ?? null, null);
+    assert.equal(rowsByVideoId.get(1)?.cardsPerHour ?? null, null);
+    assert.equal(rowsByVideoId.get(1)?.tokensPerMin ?? null, null);
   } finally {
     db.close();
     cleanupDbPath(dbPath);