fix(add): walk to the target year when adding years - #479

add(n, 'year') moves the epoch by n * 365.05 days, nudges it by a
guessed number of leap days, then walks the month and date back.
The year itself was never set, so when the guess fell short or went
too far the result kept the wrong year:

  spacetime('2023-01-01').add(11, 'year')  // 2033-01-01, not 2034
  spacetime('2040-12-31T22:26').add(2, 'year')  // 2043-12-31, not 2042

From Feb 29 the month-length clamp also used the starting leap year,
so walkTo() was asked for Feb 29 in a common year and gave up
part-way:

  spacetime('2024-02-29').add(13, 'year')  // 2037-02-25, not 2037-02-28

Set the target year on the wanted fields, so the year is walked to
directly and Feb 29 is clamped against the year we land in.

Co-Authored-By: Claude Opus 5.5 <[email protected]>