fix: derive short years from the full calendar year

[xiehuanyi]

{YY} currently uses Date.getYear(), which returns the year minus 1900. For many earlier dates the negative value is sliced incorrectly: year 1801 formats as 99, and year 1899 formats as -1, instead of 01 and 99.

Use getFullYear() and let the existing two-character formatter select the year suffix. Modern dates and custom formatter overrides retain their behavior.

Validation:

• Five historical-year assertions fail before the fix (24/29 assertions pass).
• npm test passes all 29 assertions on local Node 22 and every CI runtime: Node 6, 12 and 18.
• Regression covers early years, dates before 1900, the 1900 boundary and a modern date.
• git diff --check passes.

AI assistance: Codex assisted with discovery, implementation and validation. Tests cover nonnegative calendar years; formatting of dates before the common era is unchanged.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.