.format('YYYY') returns less than 4 digits for years < 1000

According to the [documentation](https://day.js.org/docs/en/parse/string-format#list-of-all-available-parsing-tokens), `YYYY` should return the 4-digit year. However, if the year is smaller than 1000 it returns a year with less than 4 digits.

For example, `dayjs('0202-01-01').format('YYYY-MM-DD')` returns `202-01-01`, which is to my understanding an invalid ISO date. It should return `0202-01-01` instead.

Reproduce the reported dayjs('0202-01-01').format('YYYY-MM-DD') result, then locate the YYYY token formatting path in the Day.js codebase. Add a regression test covering years below 1000 and verify that the formatted result is 4 digits while the existing formatting behavior remains intact.

Written by the indexing model from the issue text.