Scaffold .NET client projects

docs/clients-dotnet-csharp-design.md

@@ -16,6 +16,7 @@ Recommended layout:
 
 ```text
 clients/dotnet/
+  MxGateway.Client.sln
   MxGateway.Client/
     MxGateway.Client.csproj
     GatewayClient.cs
@@ -41,6 +42,12 @@ Target framework:
 <TargetFramework>net10.0</TargetFramework>
 ```
 
+The scaffold uses a project reference to
+`src/MxGateway.Contracts/MxGateway.Contracts.csproj` for generated protobuf and
+gRPC types. `clients/dotnet/generated` remains reserved for client-local
+generator output if the .NET client later needs to decouple from the contracts
+project.
+
 Expected packages:
 
 - `Grpc.Net.Client`