docs(worker): record the SemaphoreSlim/STA dispatch invariant + single-waiter contract; harden fake

src/ZB.MOM.WW.MxGateway.Worker.Tests/TestSupport/FakeRuntimeSession.cs

@@ -303,6 +303,17 @@ internal sealed class FakeRuntimeSession : IWorkerRuntimeSession
 
         if (BackingQueue is not null)
         {
+            if (WaitForEventsOnSignalOnly)
+            {
+                // The two are mutually exclusive: a real backing queue owns its own signal and
+                // always honours the timeout, so silently letting it win would leave a
+                // signal-only test passing on a fallback tick — exactly the false green the
+                // option exists to rule out.
+                throw new InvalidOperationException(
+                    "FakeRuntimeSession cannot combine BackingQueue with WaitForEventsOnSignalOnly: "
+                    + "the backing queue honours the fallback timeout, which defeats the signal-only wait.");
+            }
+
             // Tests that drive a real queue enqueue into it directly, so the real queue owns the
             // wake signal too.
             return BackingQueue.WaitForEventsAsync(timeout, cancellationToken);