Key MMTTY the way N1MM does, with the control's PTT property

Nothing was ever transmitted. SetMmttyPTT does not start a transmission: N1MM
calls it with 1 to stop once the buffer is empty, which is its XmitOff, and with
0 to stop now, which is its AbortXmit. A transmission starts by setting the
control's PTT property, in XmitOn.

So the bridge learns `key <0|1>` for that property, and MmttyEngine now keys
with it, ends a message with SetMmttyPTT(1) so the buffer still goes out, and
aborts with SetMmttyPTT(0). This is the digital {TX} in the entry window and the
digital window as well as the probe: none of them could key the engine before.

The probe checks that the engine keys before it measures anything, and stops
with a plain statement if it does not, rather than reporting numbers from an
engine sitting still. It also asks a new question: whether the engine holds a
word until the space after it, which is MMTTY's Way to send. Received characters
are marked as noise while the engine is not transmitting, since a machine with a
sound card decodes the band all the way through the run.

The first run on a real engine says MMTTY 1.70 connects, the control answers
TxBufLen and refuses NotAProperty with DISP_E_UNKNOWNNAME. What TxBufLen counts
is still open: read while nothing was transmitting it rose over time and rose by
four after four backspaces, which is not what characters-left would do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RtspmWmS7f8kUvcyaHpRWZ

tools/Nonemm.EngineProbe/Program.cs

@@ -79,13 +79,14 @@ public static class Program
         return 0;
     }
 
-    /// The engine is left keyed if the probe stopped partway through, so PTT is
-    /// dropped before the engine is, whatever happened.
+    /// The engine is left keyed if the probe stopped partway through, so the
+    /// transmission is aborted before the engine is shut down, whatever
+    /// happened.
     private static async Task Shutdown(MmttyEngine engine, ProbeLog log)
     {
         try
         {
-            await engine.SetPttAsync(false).ConfigureAwait(false);
+            await engine.AbortAsync().ConfigureAwait(false);
             await engine.StopAsync().ConfigureAwait(false);
         }
         catch (Exception problem)