Mark a dupe in the contact type column, as N1MM does

N1MM writes a contact type on every contact it logs: `D` for a station
already worked, `N` for one the entry may not work at all, and empty for an
ordinary contact. Nothing here wrote the column, so a dupe counted as a
contact worth no points instead of dropping out of the score, and the two
places that read the column did nothing: the QTC pile offered dupes that
N1MM leaves out, and the statistics window could not leave dupes out either.

The marking is in one place, `ContestLog.ApplyVerdict`, and every contest is
marked the same way. What changes between contests is what counts as a dupe
and which stations are workable, and both come from the contest, as they do
in N1MM's `Contact.IsDupe`. No contest module in N1MM marks differently. Our
own callsign is not workable either, which is what N1MM adds on top of the
contest rule.

Two details follow N1MM as well: QTC traffic keeps an empty column, because
its `IsDupe` only marks a row while the program is logging contacts; and a
mark that came with the contact stands, because N1MM marks a contact when it
logs or edits one and never clears marks in a pass over the log. Editing a
contact clears the column so it is worked out again, which is what N1MM does
at the same point: an edit can move a contact to a band where it is no
longer a dupe.

A dupe is now out of the score altogether — contacts, points and multipliers
— which is N1MM's `ScoreClass.AddContact`. The QSO count still counts WAE
QTC rows as contacts; that is the other half of the scoring audit's item 14.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0157i7PiZyqBkquxS9A6FTdQ

src/Nonemm.Contests/Rules/OceaniaDx.cs

@@ -61,7 +61,7 @@ public sealed class OceaniaDx : Contest
             ]);
 
     /// One side of every contact has to be in Oceania.
-    private static bool IsWorkable(QsoContext qso) =>
+    public bool IsWorkable(QsoContext qso) =>
         qso.Continent == "OC" || qso.Me.Continent == "OC";
 
     private static int PointsOn(Band? band) => band?.Name switch