Implement graceful worker shutdown

docs/gateway-process-design.md

@@ -175,6 +175,12 @@ Behavior:
 `CloseSession` should be idempotent. Closing an already closed session should
 return a successful close result with the final known state.
 
+`WorkerClient.ShutdownAsync` sends `WorkerShutdown`, waits for the worker read,
+write, and heartbeat loops to stop, and waits for the launched worker process to
+exit within the same shutdown timeout. If the pipe loops or process exit exceed
+the timeout, the close operation fails with `ShutdownTimeout`; `GatewaySession`
+then kills the worker process tree before surfacing the close failure.
+
 ### Invoke
 
 `Invoke` forwards one MXAccess command to the worker that owns the session.
@@ -515,6 +521,11 @@ It handles:
 The write loop should fail the session if a pipe write fails outside normal
 shutdown.
 
+During shutdown the worker client treats `WorkerShutdownAck` as the protocol
+close signal, but the process handle remains authoritative for process lifetime.
+The client waits for both the protocol close and process exit before reporting a
+clean shutdown to `GatewaySession`.
+
 ## Command Correlation
 
 Each command gets: