Resolve Worker.Tests-003..007 code-review findings

Worker.Tests-003: removed the wall-clock `Elapsed < 2s` assertion from
InvokeAsync_WakesIdlePumpForQueuedCommand; the awaited completion against a
30s idle period already proves the wake event drove dispatch.

Worker.Tests-004: MxAccessStaSession.Dispose now joins the alarm poll task
after cancelling the CTS (consistent with ShutdownGracefullyAsync), and
Dispose_StopsAlarmPollLoop asserts deterministically instead of via Task.Delay.

Worker.Tests-005: undisposed MemoryStream instances across the frame-protocol
and pipe-session tests are now `using` declarations.

Worker.Tests-006: Dispose_StopsAlarmPollLoop now constructs MxAccessStaSession
with `using` so a failed assertion cannot leak the STA poll loop.

Worker.Tests-007: docs/WorkerFrameProtocol.md verification section corrected
to target MxGateway.Worker.Tests / MxGateway.Worker with -p:Platform=x86.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Worker/MxAccess/MxAccessStaSession.cs

@@ -580,13 +580,27 @@ public sealed class MxAccessStaSession : IWorkerRuntimeSession
 
         RequestShutdown();
 
-        // Cancel and discard the STA poll loop.
+        // Cancel the STA poll loop and join it before disposing the alarm
+        // handler. Joining (rather than discarding alarmPollTask) makes the
+        // stop deterministic: once Dispose returns, no further PollOnce calls
+        // can be in flight, so callers and tests can rely on a frozen poll
+        // count instead of an elapsed-time "no further polls" window.
         CancellationTokenSource? pollCtsToDispose = alarmPollCts;
+        Task? pollTaskToJoin = alarmPollTask;
         alarmPollCts = null;
         alarmPollTask = null;
         if (pollCtsToDispose is not null)
         {
             try { pollCtsToDispose.Cancel(); } catch { }
+            if (pollTaskToJoin is not null)
+            {
+                try
+                {
+                    pollTaskToJoin.Wait(TimeSpan.FromSeconds(5));
+                }
+                catch (AggregateException) { }
+                catch (ObjectDisposedException) { }
+            }
             try { pollCtsToDispose.Dispose(); } catch { }
         }