duration humanize ignoring leap years

**Describe the bug**
I called `dayjs.duration("P9999Y").humanize()` and was returned `9992 years`
If I call `dayjs.duration("P9999Y").asYears()` then I get 9999.

**Expected behavior**
Expected that on large durations then `humanize` returns the same number of years as `.asYears()`.

**Information**
- Day.js Version: v1.11.21
- OS: Mac OS
- Browser: Brave v1.93.132
- Time zone: BST

Start by reproducing the `dayjs.duration("P9999Y").humanize()` and `.asYears()` examples in the duration API. Trace how `humanize` converts large year values and handles leap years, then add regression coverage showing that the humanized result matches the duration's year value. Done means the reported case returns 9999 years without changing expected behavior for other durations.

Written by the indexing model from the issue text.