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>

tests/Nonemm.Storage.Tests/ContestInstanceTests.cs

@@ -0,0 +1,10 @@
+namespace Nonemm.Storage.Tests;
+
+public class ContestInstanceTests
+{
+    /// N1MM answers "Invalid Overlay Category:" and will not open a contest
+    /// whose overlay column is empty, so an entry with no overlay says "N/A".
+    [Fact]
+    public void OverlayCategoryDefaultsToNotApplicable() =>
+        Assert.Equal("N/A", new ContestInstance { ContestNumber = 1, ContestName = "CQWW" }.OverlayCategory);
+}