Fix Gregorian leap-year handling

[OsirisSlain]

Fix leap-year calculations to use the Gregorian year derived from YOLD.

YOLD is Gregorian + 1166, so converting back for leap-year checks needs to subtract 1166. The old helper added 1166, which made X-Day countdown calculations drift around some leap-year boundaries.

This also makes explicit Feb 29 validation use the same no-year-zero-adjusted year that ddate stores internally, fixing BCE leap-day handling.

Examples affected:

• 29 2 -1 is now accepted as St. Tib's Day
• 29 2 -4 is now rejected
• century leap rules are applied through the shared leapp() helper