Log the time the BARTG exchange went out

The BARTG RTTY contest sends the time with the serial number, and both
stations log the time the other one sent. The entry window now has the
report, serial and time boxes N1MM's has, and the time this station sent
is stamped into the MiscText column as the exchange message goes out, or
at log time when nothing sent it. The Cabrillo line writes that time and
falls back to the time of the contact when the column is empty, which is
what N1MM writes, in N1MM's columns.

The total score was points times every multiplier. N1MM's class works
out points times the countries and areas together, times the continents.
Per-contact points and multiplier flags do not change, so the station's
2022 log still scores contact for contact as N1MM did.

N1MM has a class each for the March HF RTTY contest and the September
sprint. This program has one contest holding the March rules under the
sprint's name, and docs/unfinished.md now says so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.App/Windows/EntryWindow.Macros.cs

@@ -79,6 +79,11 @@ public sealed partial class EntryWindow
             Status("no keyer — Config ▸ Keyer");
             return;
         }
+        if (index == Esm.Exchange)
+        {
+            // the BARTG contest sends the time, and logs the one it sent
+            Logging.StampSentTime();
+        }
         string text = prefix + plan.Text;
         // the box has to point at this radio before the key does
         await session.PointTransmitAtAsync(radioNumber);