EPPlusSoftware/EPPlus — reverse-engineered prompt

Build me a C# .NET library for working with Excel .xlsx spreadsheets, similar to EPPlus 8. I want developers to be able to create a workbook, open an existing file, add worksheets, write and read cell values, and save the file without needing Excel installed. Make the API feel simple, with an ExcelPackage style object around a workbook and clear examples for common use.

Please also add the required license setup before use. Support commercial and noncommercial modes, settable in code, appsettings, web config, or an environment variable called EPPlusLicense. If the license is missing, give a helpful error that tells the user what to do.

Set it up as a real .NET project with tests, basic docs, and a small sample console app that creates a workbook. Keep it compatible with modern .NET and older .NET where reasonable. Look up current docs online if you need to.