Optional kotlinx.serialization KSerializers for the nepali-date-picker-core data types (SimpleDate, SimpleTime, CustomCalendar, NepaliMonthCalendar). Bring this in when you want to serialize Nepali dates over JSON / Protobuf / CBOR via Ktor, Room TypeConverters, DataStore, or any other kotlinx-serialization consumer. The core module remains annotation-free - no kotlinx-serialization dependency leaks into projects that do not depend on this artifact.