Extends Clock_IncMS()'s existing sec->min->hour cascade to also roll
day->month->year (with leap-year Feb 29 via Days_In_Month()), and adds
Clock_RecalcWeekday() computing current_weekday via Zeller's congruence
whenever the date changes, rather than trusting PCF8563's own weekday
register (which would drift if the date is edited manually). Verified
the formula against 2026-07-31, a known Friday.