Facepunch/Facepunch.Steamworks — reverse-engineered prompt

Build me a clean C# Steamworks library that feels easy to use in a normal game project and also works well in Unity, including IL2CPP. I want it to be a single managed DLL with no extra native dependency beyond Steam itself, and it should run on Windows, Linux, and macOS.

It should cover the main stuff a game would need, like initializing a client or server, getting the current user info, friends list and messages, app info, avatars, internet server queries, achievements, voice data, auth tickets for validating players, workshop download and item queries, publishing workshop files, cloud save file read and write, and Steam inventory access. Please make the API feel like real C#, with async support for call results and event style callbacks where that makes sense.

Include a few small examples or demo code for the common flows, plus some basic tests so it is easier to trust. If anything is unclear, look up the current Steamworks docs online and fill in the gaps sensibly.