Take serial numbers from a pool the whole entry shares

Each station worked out its next number as the highest in its log plus one,
so two operators typing at the same moment both sent the same number and
one of the two contacts was wrong in the sponsor's log.

SerialNumberPool holds the highest number taken per band, which is N1MM's
QSONumbers array. A number is taken at the first character of the callsign,
as N1MM takes it, and ReserveNr tells the other stations before the contact
is worked. A number at or below one this station has used is answered with
the whole pool, N1MM's QSONRS, and the station that is behind counts on from
there. The pool also goes out when a contest is opened and when the numbers
are started again.

Contest.NumbersByBand says whether the numbers run per band or as one
sequence for the station. CQ WPX runs one per band for a multi-two and a
multi-multi; every other contest here runs one sequence. The band index in
the message is N1MM's, so the array lines up with theirs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015NZkFxHde1iifAQemFTHxf

src/Nonemm.Session/EntryFields.cs

@@ -16,7 +16,24 @@ public sealed class EntryFields
 
     public IReadOnlyList<ExchangeField> Exchange { get; }
 
-    public string Call { get; set; } = "";
+    /// Raised when the callsign box goes from empty to holding something. That
+    /// is where N1MM takes the serial number for the contact, so the number is
+    /// out on the network before the contact is worked.
+    public event EventHandler? CallStarted;
+
+    public string Call
+    {
+        get;
+        set
+        {
+            bool wasEmpty = field.Trim().Length == 0;
+            field = value;
+            if (wasEmpty && field.Trim().Length > 0)
+            {
+                CallStarted?.Invoke(this, EventArgs.Empty);
+            }
+        }
+    } = "";
 
     /// 0 is the callsign box; 1 and up are the exchange boxes in order.
     public int Focus { get; private set; }