Incorrect week() for UTC time zone

**Describe the bug**
Weeks for UTC time zone (+0) seem to be off.
May 22nd is Thursday, 21st ISO week.
```
console.log(dayjs.tz.guess()) // Europe/Amsterdam
console.log(dayjs('2025-05-22T12:00:00Z').tz('America/New_York').week()) // 21
console.log(dayjs('2025-05-22T12:00:00Z').tz('UTC').week()) // 25
console.log(dayjs('2025-05-22T12:00:00Z').tz('Portugal').week()) // 25
console.log(dayjs('2025-05-22T12:00:00Z').tz('Europe/Riga').week()) // 21
```

**Expected behavior**
Regardless of the time zone, week number should be 21 in this case and certainly not 4 weeks off.

**Information**
- Day.js Version: 1.11.13
- OS: Debian 12 x86_64
- Browser Brave 1.78.102 Chromium: 136.0.7103.113 (also tested in node v22.15.0 and on Firefox)
- Time zone: Europe/Amsterdam

P.S. Thanks for your kind work.

Reproduce the issue with the provided dayjs.tz and week() calls, then trace the timezone and week-number entry points implementing those APIs. Compare UTC, Portugal, and Riga with the expected ISO week 21, and finish with a regression check showing equivalent zones return the correct result.

Written by the indexing model from the issue text.