fix: parseJSONDate incorrectly adding 1900 to years 0-99

[ascott18]

Why

parseJSONDate was returning the wrong year for dates with a year in the range 0-99. For example, parseJSONDate("0001-01-01", "date") produced Tue Jan 01 1901 instead of year 1. This is because JavaScript's Date constructor (and Date.UTC) interpret a year of 0-99 as 1900 + year.

Approach

After constructing the Date, we now shift the result back by exactly 1900 years, but only when the parsed year falls in the 0-99 range. The shift is relative (getFullYear() - 1900) rather than an absolute setFullYear(parsedYear). This matters because the constructor may normalize the date across a year boundary (e.g. a timezone offset that pushes the UTC instant into the previous year); a relative shift preserves that normalization, whereas an absolute set would clobber it and produce an off-by-one-year result.

The fix is applied consistently across all three relevant branches: date-only, datetime without offset, and datetime with a Z/explicit offset. The time-only branch is unaffected (it already uses the current year by design).

Testing

Coverage was added through the existing convertToModel/mapToModel date-mapping table in model.toModel.spec.ts (runs against both mapping variants), covering:

• Small years across all three branches (0001-01-01, 0099-06-15, 0001-01-01T14:00:00, 0001-06-15T12:00:00Z)
• Leap day in a small year (0004-02-29)
• A normal year whose offset crosses back into the prior year, which must NOT be clobbered (2020-01-01T00:30:00+01:00)
• A small year whose offset crosses back into year 0, validating the relative shift (0001-01-01T00:30:00+01:00)
• The year < 100 boundary (0100-01-01), which must be left untouched

All 668 coalesce-vue tests pass and tsc is clean.