`utcOffset()` returns incorrect result after adding or subtracting hours to a value.

**Describe the bug**

```js
const dayjs = require('dayjs'); dayjs.extend(require('dayjs/plugin/utc')); dayjs.extend(require('dayjs/plugin/timezone'));

const t1 = dayjs.tz('2022-03-13T00:00:00', 'America/Montreal');
const t2 = dayjs.tz('2022-03-13T04:00:00', 'America/Montreal');
console.log(t1.utcOffset()); // -300, which is correct
console.log(t2.utcOffset()); // -240, which is correct, because we crossed a DST boundary

const t3 = t1.add(3, 'hours');
console.log(t3.valueOf() === t2.valueOf()); // True - midnight plus three hours is 4am, because of DST.
console.log(t3.utcOffset()); // -300, which is incorrect - should be -240.
```

**Expected behavior**

When moving a date between DST periods, we expect the utcOffset to be updated (this is how moment.js works).

**Information**

- Day.js Version: 1.10.8
- OS: MacOS
- Browser: Node 16.14.0
- Time zone: America/Montreal

Start by running the reported Node.js reproduction with the utc and timezone plugins for America/Montreal, then trace the utcOffset behavior used by add(). Add a regression test covering the DST boundary; done means the added value reports the same offset as the equivalent parsed time (-240).

Written by the indexing model from the issue text.