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

bridge/main.cpp

@@ -145,6 +145,14 @@ void send(const std::string& text) {
     VariantClear(&argument);
 }
 
+// N1MM's {TX}: the control's own PTT property, which is what starts a
+// transmission. SetMmttyPTT is the other direction, and only that.
+void key(bool on) {
+    report("PTT", control->putBool(L"PTT", on));
+}
+
+// 1 stops when the buffer is empty, which is N1MM's XmitOff; 0 stops now,
+// which is its AbortXmit.
 void setPtt(long on) {
     VARIANT argument;
     VariantInit(&argument);
@@ -177,6 +185,8 @@ void act(const std::string& text) {
         open(line);
     } else if (line.verb == "send") {
         send(line.field(0));
+    } else if (line.verb == "key") {
+        key(line.number(0) != 0);
     } else if (line.verb == "ptt") {
         setPtt(line.number(0));
     } else if (line.verb == "buffer") {