Two entry windows and an SO2R box

LoggingSession mixed two things: the contest, which one station has one of,
and what the operator is typing, which belongs to a radio. It is now
ContestSession and RadioPosition. Two positions share one log, one score and
one run of serial numbers, so a station worked on radio 1 is a dupe on radio 2.
Frequency, mode, split, what is typed and whether you are running belong to
each radio on its own.

A second radio opens a second entry window. It has no menu of its own: it is
another view of the same contest, not another program. Only the window for
radio 1 opens the database and the connections.

Ctrl+Tab moves the operator to the other radio and the keyboard follows.
Ctrl+Shift+Tab puts both radios in the headphones.

OtrspBox speaks OTRSP to an SO2R box over a serial port: TX1/TX2 for the key,
RX1/RX2 and RX1S for the headphones, AUXnnn for an output line. It follows the
active radio, and every message points the box at this radio before keying, so
a message cannot go out of the radio the operator has just left. A command that
would change nothing is not sent, because the box works relays; N1MM does the
same. The box takes a Stream, so what it sends is tested without a serial port.

Still missing: alternating CQ, and voice keying on the second radio.

Looked at under Xvfb with two fake radios, one of them split: both windows up,
radio 1 showing 14008.00 into 14020.0, radio 2 on 14030.00, Ctrl+Tab moving the
keyboard between them.

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

src/Nonemm.Rig/So2rBox.cs

@@ -0,0 +1,19 @@
+namespace Nonemm.Rig;
+
+/// The box between the operator and two radios: it decides which radio the
+/// transmitter and the key reach, and which one the headphones hear. Without a
+/// box the operator does that with switches, and the logger only has to keep
+/// track of which radio it is on.
+public interface So2rBox : IDisposable
+{
+    /// Points the transmitter and the key at one radio.
+    Task SetTransmitAsync(int radioNumber, CancellationToken cancellation = default);
+
+    /// Points the headphones at one radio, or at both when `bothRadios` is set,
+    /// which is how an operator listens to the second radio while the first is
+    /// sending.
+    Task SetReceiveAsync(int radioNumber, bool bothRadios = false, CancellationToken cancellation = default);
+
+    /// Sets a box output, which is usually an antenna or band-decoder line.
+    Task SetAuxiliaryAsync(int radioNumber, int code, CancellationToken cancellation = default);
+}