Write what N1MM needs before it will open a log

Checked by opening a log this program wrote in N1MM 1.0.11031. Three things it
rejected:

- a log naming a contest with no row in the Contest table throws "No current
  row" in Contest.FromRow, so the definition row is written whenever a contest
  is opened;
- an empty overlay category is answered with "Invalid Overlay Category:", so an
  entry with no overlay now says "N/A", and the dialog offers N1MM's list;
- the sent exchange omits the report, which is what N1MM's own contest dialog
  asks for.

With those three fixed N1MM opens the log and shows the contacts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Nonemm.Contests/Rules/CqWpx.cs

@@ -40,7 +40,9 @@ public sealed class CqWpx : Contest
 
     public IReadOnlyList<ModeCategory> Modes => [mode];
 
-    public string SentExchangeFor(StationInfo me) => DefaultReport();
+    /// The serial number is generated per contact, so the stored exchange is
+    /// the number the contest starts at.
+    public string SentExchangeFor(StationInfo me) => "001";
 
     public int PointsFor(QsoContext qso)
     {
@@ -89,5 +91,4 @@ public sealed class CqWpx : Contest
         _ => "RTTY",
     };
 
-    private string DefaultReport() => mode == ModeCategory.Phone ? "59" : "599";
 }