V2: Calendar architecture cleanup

[Nielsvanpach]

Summary

• Extract duplicated getSingleDayHoliday, getMultiDayHoliday, createPeriod, and getOverlapping methods from IslamicCalendar and IndianCalendar into a shared ResolvesCalendarDates trait
• Simplify Observable trait: weekendToNextMonday() and sundayToNextMonday() now accept only CarbonInterface (removed dead string handling and $year parameter)
• Change calendar lookup constants on Albania, Turkey, and India from public const to protected const
• Add @mixin Country annotations to ChineseCalendar and NepaliCalendar for PHPStan

Breaking changes

• Observable::weekendToNextMonday() and sundayToNextMonday() signature changed from (string|CarbonInterface $date, int $year) to (CarbonInterface $date)
• Calendar lookup constants (eidAlFitr, eidAlAdha, holiHolidays, etc.) changed from public to protected