Add Polly resilience policies

docs/clients-dotnet-csharp-design.md

@@ -110,10 +110,17 @@ public sealed class MxGatewayClientOptions
     public string? ServerNameOverride { get; init; }
     public TimeSpan ConnectTimeout { get; init; } = TimeSpan.FromSeconds(10);
     public TimeSpan DefaultCallTimeout { get; init; } = TimeSpan.FromSeconds(30);
+    public MxGatewayClientRetryOptions Retry { get; init; } = new();
     public ILoggerFactory? LoggerFactory { get; init; }
 }
 ```
 
+The .NET client applies a bounded Polly retry policy only to idempotent calls:
+`CloseSession` and diagnostic `Invoke` commands such as `Ping`,
+`GetSessionState`, and `GetWorkerInfo`. It does not retry `OpenSession`, event
+streams, writes, secured writes, authentication, registration, item management,
+or subscription changes because those calls can partially succeed in MXAccess.
+
 API key may be loaded from `MXGATEWAY_API_KEY` by the CLI, not implicitly by the
 library constructor unless a helper explicitly says it does that.