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>

tests/Nonemm.Core.Tests/GridSquareTests.cs

@@ -0,0 +1,46 @@
+using Nonemm.Core;
+
+namespace Nonemm.Core.Tests;
+
+public class GridSquareTests
+{
+    [Fact]
+    public void FourCharacterSquareCentresOnTheSquare()
+    {
+        Assert.True(GridSquare.TryParse("JN88", out GridSquare grid));
+        Assert.Equal(48.5, grid.Latitude, 3);
+        Assert.Equal(17.0, grid.Longitude, 3);
+    }
+
+    [Fact]
+    public void SixCharacterSquareIsMorePrecise()
+    {
+        Assert.True(GridSquare.TryParse("FN31pr", out GridSquare grid));
+        Assert.Equal(41.7, grid.Latitude, 1);
+        Assert.Equal(-72.7, grid.Longitude, 1);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("JN8")]
+    [InlineData("SN89")]
+    [InlineData("JNXY")]
+    public void MalformedLocatorIsRejected(string text) =>
+        Assert.False(GridSquare.TryParse(text, out _));
+
+    [Fact]
+    public void DistanceIsTheGreatCircleBetweenSquareCentres()
+    {
+        Assert.True(GridSquare.TryParse("JO99", out GridSquare jo99));
+        Assert.True(GridSquare.TryParse("IO91", out GridSquare io91));
+        Assert.InRange(jo99.DistanceTo(io91), 1525, 1540);
+    }
+
+    [Fact]
+    public void BearingIsZeroDueNorth()
+    {
+        Assert.True(GridSquare.TryParse("JO99", out GridSquare south));
+        Assert.True(GridSquare.TryParse("JP90", out GridSquare north));
+        Assert.Equal(0, south.BearingTo(north), 6);
+    }
+}