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.Storage/ContestInstance.cs

@@ -22,7 +22,9 @@ public sealed record ContestInstance
 
     public string ModeCategory { get; init; } = "";
 
-    public string OverlayCategory { get; init; } = "";
+    /// N1MM rejects a contest whose overlay is empty, so an entry with no
+    /// overlay says so with "N/A".
+    public string OverlayCategory { get; init; } = "N/A";
 
     public string StationCategory { get; init; } = "";