Issue #11: implement gateway workerclient

docs/gateway-process-design.md

@@ -411,7 +411,7 @@ session ids as protocol faults and close the session.
 
 `WorkerClient` is the gateway-side object that owns one worker connection.
 
-Suggested public shape:
+Current public shape:
 
 ```csharp
 public interface IWorkerClient : IAsyncDisposable
@@ -419,6 +419,7 @@ public interface IWorkerClient : IAsyncDisposable
     string SessionId { get; }
     int? ProcessId { get; }
     WorkerClientState State { get; }
+    DateTimeOffset LastHeartbeatAt { get; }
 
     Task StartAsync(CancellationToken cancellationToken);
     Task<WorkerCommandReply> InvokeAsync(
@@ -438,12 +439,17 @@ Internally it owns:
 - pipe stream,
 - read loop,
 - write loop,
-- bounded outbound command/control channel,
+- outbound command/control channel serialized by the write loop,
 - bounded inbound event channel,
 - pending command dictionary keyed by correlation id,
 - heartbeat monitor,
 - terminal fault source.
 
+`StartAsync` sends `GatewayHello`, verifies the `WorkerHello` protocol version
+and nonce, waits for `WorkerReady`, and only then exposes `Ready` state. The
+read loop starts after readiness so the handshake has a single owner for its
+ordered frames.
+
 ### Read Loop
 
 The read loop: