/// A solar-calendar date using tyme-compatible Julian/Gregorian reform semantics.
///
/// Conversion APIs support solar years `1..=9999`. Dates before
/// `1582-10-15` use Julian-calendar semantics. The historical Gregorian reform
/// gap `1582-10-05..=1582-10-14` is invalid, matching tyme4rs.
/// A date in the Chinese lunar calendar.
///
/// Lunar-month fact APIs support lunar years `-1..=9999`. `lunar_to_solar`
/// additionally requires the resulting solar date to fall within supported
/// solar years `1..=9999`.