Implement .NET gateway client sessions

clients/dotnet/README.md

@@ -7,9 +7,9 @@ CLI, and unit tests.
 
 | Project | Purpose |
 |---------|---------|
-| `MxGateway.Client` | .NET 10 library entry point and raw gRPC client access. |
+| `MxGateway.Client` | .NET 10 library entry point, raw gRPC calls, and session helpers. |
 | `MxGateway.Client.Cli` | Test CLI for smoke and diagnostic commands. |
-| `MxGateway.Client.Tests` | Unit tests for the scaffold and generated contract wiring. |
+| `MxGateway.Client.Tests` | Unit tests for client options, generated contract wiring, auth metadata, session helpers, cancellation, and event streaming. |
 
 The projects reference `src/MxGateway.Contracts/MxGateway.Contracts.csproj` so
 the client compiles against the same generated protobuf and gRPC types as the
@@ -22,3 +22,44 @@ future client build switches to client-local `Grpc.Tools` generation.
 dotnet build clients/dotnet/MxGateway.Client.sln
 dotnet test clients/dotnet/MxGateway.Client.sln --no-build
 ```
+
+## Client Usage
+
+`MxGatewayClient` opens a gRPC channel to the gateway and attaches the API key
+to every unary and streaming call as `authorization: Bearer <api-key>`.
+Cancellation tokens passed to the public methods flow to the generated gRPC
+call. Client-side cancellation stops waiting for the gateway response; it does
+not abort an MXAccess COM call that is already executing inside a worker.
+
+```csharp
+await using MxGatewayClient client = MxGatewayClient.Create(
+    new MxGatewayClientOptions
+    {
+        Endpoint = new Uri("http://localhost:5000"),
+        ApiKey = apiKey,
+    });
+
+MxGatewaySession session = await client.OpenSessionAsync();
+try
+{
+    int serverHandle = await session.RegisterAsync("sample-client");
+    int itemHandle = await session.AddItemAsync(
+        serverHandle,
+        "Area001.Pump001.Speed");
+
+    await session.AdviseAsync(serverHandle, itemHandle);
+}
+finally
+{
+    await session.CloseAsync();
+}
+```
+
+Use `OpenSessionRawAsync`, `CloseSessionRawAsync`, `InvokeAsync`, and
+`StreamEventsAsync` when tests or parity tools need direct generated protobuf
+messages. `MxGatewaySession.OpenSessionReply` keeps the raw session-open reply
+available, and command helpers have `*RawAsync` variants when callers need the
+complete `MxCommandReply`.
+
+`MxGatewaySession.CloseAsync` is explicit and idempotent. Repeated calls return
+the first `CloseSessionReply` instead of sending another close request.