Add the session, radio control, bandmap and cluster

Nonemm.Session holds what the operator is typing, what the log says about it and
what happens on Enter, with no UI toolkit behind it. Nonemm.Rig talks to
hamlib's rigctld and reconnects on its own. Nonemm.Spotting reads DX cluster
lines into a bandmap that drops spots after an hour.

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

src/Nonemm.Spotting/Spot.cs

@@ -0,0 +1,15 @@
+using Nonemm.Core;
+
+namespace Nonemm.Spotting;
+
+/// One station on the bandmap.
+public sealed record Spot(
+    Callsign Call,
+    Frequency Frequency,
+    DateTime AtUtc,
+    SpotSource Source,
+    string Spotter = "",
+    string Comment = "")
+{
+    public Band? Band => Bands.ForFrequency(Frequency);
+}