Add the core, contest, storage and format layers

Frequencies, bands, modes, callsigns, grid squares and the country file live in
Nonemm.Core. Nonemm.Contests holds the scoring engine and CQ WW and CQ WPX.
Nonemm.Storage writes N1MM's DXLOG schema, and Nonemm.Formats writes Cabrillo
3.0 and reads and writes ADIF.

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

src/Nonemm.Contests/ExchangeSlot.cs

@@ -0,0 +1,20 @@
+namespace Nonemm.Contests;
+
+/// Which field of a QSO an exchange box fills in. The names match the log
+/// columns so an exchange lands in the same place N1MM would put it.
+public enum ExchangeSlot
+{
+    ReceivedReport,
+    SerialNumber,
+    Zone,
+    Section,
+    Check,
+    Precedence,
+    Exchange1,
+    MiscText,
+    Name,
+    Qth,
+    GridSquare,
+    Power,
+    Comment,
+}