Add direct C# Environment API

[deltathedumb]

Adds a first-class managed-host ABI directly to portapy.dll and a safe C# wrapper.

Supported call shape:

using var env = new PortaPy.Environment();
env.Add(helloWorld);
env.Execute("helloWorld()");

The DLL owns runtime creation, callback IDs, global injection, UTF-16 conversion, dispatch, and cleanup. The C# wrapper roots delegates and propagates managed callback exceptions safely.

Validation includes Linux/Windows C ABI hosts and a Windows .NET 8 smoke test running the exact managed flow.