Inconsistent results on 25 hour day after time change

**Describe the bug**

```javascript
const time = 1_762_066_800_000;
const timezone = 'US/Pacific';

const a = dayjs.tz(time, timezone)
.add(1, 'day');

const b = dayjs.tz(
dayjs.tz(time, timezone).add(1, 'day').valueOf(),
timezone
);

const c = dayjs.tz(
dayjs.tz(time, timezone).add(1, 'day').startOf('millisecond').valueOf(),
timezone
);

console.log('a', a.valueOf(), a.format('YYYY-MM-DD HH:mm:ss'));
console.log('b', b.valueOf(), b.format('YYYY-MM-DD HH:mm:ss'));
console.log('c', c.valueOf(), c.format('YYYY-MM-DD HH:mm:ss'));
```

Output:

```javascript
a 1762153200000 2025-11-03 00:00:00
b 1762153200000 2025-11-02 23:00:00
c 1762156800000 2025-11-03 00:00:00
```

**Expected behavior**

I expect all 3 results to be:

```javascript
a 1762156800000 2025-11-03 00:00:00
b 1762156800000 2025-11-03 00:00:00
c 1762156800000 2025-11-03 00:00:00
```

**Information**
- Day.js Version 1.11.19
- OS: MacOS 15.5
- Browser Node 22
- Time zone: US/Pacific

Start by running the supplied reproduction with Node 22 and Day.js 1.11.19, focusing on the dayjs.tz(), add(), valueOf(), and startOf('millisecond') paths around the US/Pacific time change. Done means all three results produce valueOf() 1762156800000 and format as 2025-11-03 00:00:00, with a regression test covering the case.

Written by the indexing model from the issue text.