Fix !!timestamp resolution for years 0000-0099

[sarathfrancis90]

While round-tripping Date values through YAML11_SCHEMA I hit a case where a date in years 1–99 throws on reload:

const d = new Date(0); d.setUTCFullYear(50, 0, 1)
load(dump(d, { schema: YAML11_SCHEMA }), { schema: YAML11_SCHEMA })
// -> throws: cannot resolve a node with !<tag:yaml.org,2002:timestamp> explicit tag

And loading a valid low-year timestamp directly silently yields a string instead of a Date:

load('0001-01-01', { schema: YAML11_SCHEMA }) // -> '0001-01-01' (string), expected a Date

Root cause: resolveYamlTimestamp builds the date with Date.UTC(year, …), which applies JavaScript's legacy two-digit-year remap (0–99 → 1900–1999). The Date is then off by ~1900 years, so the calendar-normalization guard (getUTCFullYear() !== year) rejects the value and it falls back to a plain string. During dump that same rejection forces an explicit !!timestamp tag onto the scalar, which then can't resolve on load — hence the throw.

Fix: restore the intended year with setUTCFullYear(year, month, day) before the validity check. Passing month/day re-applies the day in the context of the real year, so invalid calendar dates like 0001-02-29 are still rejected.

Tested with npm test (all green). Extended test/core/tags/timestamp.test.mjs with low-year load cases, a leap day (0004-02-29), rejection of an invalid low-year date, and a dump → load round-trip.

[puzrin]

I understand the problem, but I don't like the implementation with a duplicated check for the year subrange. IMHO, it's worth creating a helper

function makeUtcDate (year, month, day, hour = 0, minute = 0, second = 0, fraction = 0) {
  const date = new Date(Date.UTC(year, month, day, hour, minute, second, fraction))

  // Date.UTC() treats years 0..99 as 1900..1999. Restore the parsed YAML year
  // before validating calendar normalization, e.g. reject 0001-02-29.
  date.setUTCFullYear(year, month, day)

  return date
}

That's more readable and "straight".

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Tests are bloated. It's worth just adding a couple of good samples to the existing one (0001-01-01, 0050-06-15T12:30:00Z) and 0001-02-29 to the existing test of rejected examples.