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/Multiplier.cs

@@ -0,0 +1,8 @@
+namespace Nonemm.Contests;
+
+/// One multiplier a QSO claims. `Scope` is what makes it distinct: a country
+/// counted once per band carries the band, one counted once carries nothing.
+public sealed record Multiplier(int Index, string Value, string Scope)
+{
+    public override string ToString() => Scope.Length == 0 ? Value : $"{Value}/{Scope}";
+}