fix: parse ISO dates with a year before 1000

[Osamaali313]

Summary

parse() throws on a valid, zero-padded ISO date whose year is before 1000:

parse("0087-06-15", "YYYY-MM-DD") // throws: Date (0087-06-15) does not match format (YYYY-MM-DD)
parse("0500-01-02", "YYYY-MM-DD") // throws

The cause is the four helper in src/common.ts:

/** Creates a leading zero string of 4 digits. */
export const four = (n: number) => String(n).padStart(2, "0")

It's named four and documented "4 digits", but pads to 2. It's used only in parse.ts to assemble the ISO string passed to new Date():

const isoString = `${four(Y)}-${two(M + 1)}-${two(D)}T...`
const d = new Date(isoString)
if (isFinite(+d)) return d
return invalid()

For Y < 1000, four(Y) yields a 1–3 digit year, so the string (e.g. "87-06-15T...") isn't valid ISO 8601, new Date() is Invalid Date, and parse() throws.

Fix

Pad to 4, as the helper's name and docstring intend:

export const four = (n: number) => String(n).padStart(4, "0")

This is a no-op for years ≥ 1000 (their string is already ≥ 4 chars, so padStart(4) and padStart(2) are identical), so no existing behavior changes. The library already committed to supporting non-4-digit years elsewhere (offset handling, #82); this closes the same gap in parse().

Testing

Added a regression test in parse.spec.ts (parses an ISO date whose year is before 1000). Before the fix it throws; after, it returns the correct date. The full parse suite passes (33/33) and the fix leaves every other test unchanged.

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