fix: pad full years to four digits

[xiehuanyi]

{YYYY} formats years 0 through 9 with three digits: year 1 becomes 001 instead of 0001. The built-in formatter prepends only two zeros before slicing to the token width.

Use three leading zeros, enough for the widest built-in token. Existing shorter tokens still take their required suffix width, and modern four-digit years retain their output.

Validation:

• Three new assertions fail against the original code (27/30 assertions pass).
• npm test passes all 30 assertions on local Node 22 and every CI runtime: Node 6, 12 and 18.
• Regression covers years 0, 1, 9 and the boundaries to two-, three- and four-digit years.
• git diff --check passes.

AI assistance: Codex assisted with discovery, implementation and validation. Tests cover nonnegative calendar years; custom formatters continue to control their own padding.

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