Add the Avalonia application and the station network

The entry window types contacts and colours them as they are typed; the log,
check, bandmap, score and packet windows read the same session. Contacts are
shared with the other stations of a multi-operator entry in N1MM's contact
message, so an N1MM station on the same network sees them too.

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

src/Nonemm.Session/CheckCandidate.cs

@@ -0,0 +1,25 @@
+using Nonemm.Contests;
+using Nonemm.Core.Calls;
+
+namespace Nonemm.Session;
+
+/// One callsign the check window offers, with what the log says about it so it
+/// is coloured the same as everywhere else.
+public sealed record CheckCandidate(string Call, PartialMatchQuality Quality, Verdict? Verdict);
+
+/// Where a column of candidates came from. They stay apart because they answer
+/// different questions: the log is what this station copied itself, the
+/// database is a guess about the whole world, and the bandmap is somebody
+/// else's claim.
+public enum CheckSource
+{
+    Log,
+    Database,
+    Bandmap,
+}
+
+/// One column of the check window.
+public sealed record CheckColumn(
+    CheckSource Source,
+    int Held,
+    IReadOnlyList<CheckCandidate> Candidates);