RajuPrasai/NepDate ? reverse-engineered prompt

Build me a fast, memory efficient C# library for Nepali Bikram Sambat dates that feels a lot like .NET DateOnly, but for BS dates.

I want a readonly struct that can be created from year month day, from a string, or from a normal DateTime, and it should convert back and forth between Nepali and English dates cleanly. It should support date math like adding days and months, comparisons, ranges, formatting, parsing, and helpful calendar info like month names, holidays, events, tithi, and fiscal year calculations for Nepal. Please make it work nicely with common .NET features like JSON serialization, XML, IFormattable, parsing, and type conversion.

Keep it lightweight, accurate, and easy to use, with no external dependencies. Include tests and basic docs, and if you need to check current .NET docs or best practices online, go ahead and look them up.