Add CW keying, function key messages and the documentation

The function keys send through cwdaemon or a WinKeyer, with N1MM's message
macros. Escape stops sending.

Settings are read with the reflection serializer rather than a generated one:
the generated one hands back null for every property the file leaves out
instead of the value the property is declared with, which crashed the program
the first time a new setting was added.

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

tests/Nonemm.Network.Tests/ContactMessageTests.cs

@@ -0,0 +1,59 @@
+using Nonemm.Core;
+
+namespace Nonemm.Network.Tests;
+
+public class ContactMessageTests
+{
+    private static Qso Contact() => new()
+    {
+        Id = "0123456789abcdef0123456789abcdef",
+        TimestampUtc = new DateTime(2026, 5, 30, 12, 34, 56, DateTimeKind.Utc),
+        Call = Callsign.Parse("JA1XYZ"),
+        Frequency = Frequency.FromKilohertz(14_025.5),
+        Mode = Modes.Cw,
+        ContestName = "CQWW",
+        ContestNumber = 3,
+        SentReport = "599",
+        ReceivedReport = "579",
+        Zone = 25,
+        Points = 3,
+        IsMultiplier1 = true,
+        CountryPrefix = "JA",
+        Continent = "AS",
+        WpxPrefix = "JA1",
+        IsRunQso = true,
+    };
+
+    [Fact]
+    public void ContactSurvivesTheRoundTrip()
+    {
+        Qso? read = ContactMessage.Read(ContactMessage.Write(Contact(), "DL1ABC", "PC1"));
+
+        Assert.NotNull(read);
+        Assert.Equal("0123456789abcdef0123456789abcdef", read.Id);
+        Assert.Equal("JA1XYZ", read.Call.Text);
+        Assert.Equal(14_025_500, read.Frequency.Hertz);
+        Assert.Equal(new DateTime(2026, 5, 30, 12, 34, 56, DateTimeKind.Utc), read.TimestampUtc);
+        Assert.Equal(25, read.Zone);
+        Assert.Equal("JA", read.CountryPrefix);
+        Assert.True(read.IsRunQso);
+        Assert.Equal("PC1", read.StationName);
+    }
+
+    /// A contact that arrived over the network was made somewhere else, whatever
+    /// the sender put in the field.
+    [Fact]
+    public void AContactFromTheNetworkIsNotOriginal() =>
+        Assert.False(ContactMessage.Read(ContactMessage.Write(Contact(), "DL1ABC", "PC1"))!.IsOriginal);
+
+    [Fact]
+    public void FrequenciesAreSentInUnitsOfTenHertz() =>
+        Assert.Contains("<rxfreq>1402550</rxfreq>", ContactMessage.Write(Contact(), "DL1ABC", "PC1"));
+
+    [Theory]
+    [InlineData("<radioinfo><app>N1MM</app></radioinfo>")]
+    [InlineData("not xml at all")]
+    [InlineData("<contactinfo><call></call></contactinfo>")]
+    public void AnythingThatIsNotAContactIsPassedOver(string message) =>
+        Assert.Null(ContactMessage.Read(message));
+}