feat(sat): Doppler tracking on the radio

The hard part of satellite tuning is not the arithmetic, it is deciding
who owns the dial. A tracker that forces both frequencies fights the
operator every time they turn the knob to follow a station across a
linear transponder; one that never touches the receiver leaves them
chasing a signal that slides nine kilohertz across a 70 cm pass.

So the operator owns the receiver and the tracker follows them. Every
second it asks the radio where the receiver actually is. Where it put it,
nothing has changed. Further than a dial-turn's tolerance, and the
operator has chosen a station: what they landed on is converted back into
a nominal frequency, and the transmitter is derived from that. Which is
the division of labour on a linear bird — the operator listens, the radio
does the sums.

Three ways to reach the radio, because a satellite pair is a shape of
operating rather than a manufacturer's feature. An IC-9700 or IC-9100 is
asked for its OWN satellite mode: it pairs main and sub, gives full
duplex, and keeps the dials linked the way its designers meant, which is
always better than an imitation built out of split. A Flex gets two
slices, A the downlink and B the uplink, created when missing, because
"slice B does not exist" is not something to make an operator fix at the
start of a ten-minute pass. Everything else gets the downlink, and is
told so — half the job announced beats half the job hidden.

What goes in the log is the NOMINAL pair. Two stations working each other
through a transponder read different numbers off their dials at the same
instant; the only figure they can both agree on is the transponder's own.
FREQ is the uplink and FREQ_RX the downlink — the one place a satellite
QSO differs from every other kind, and the reason FREQ alone cannot
describe one.

app_sat.go

@@ -638,6 +638,12 @@ func (a *App) GetSatelliteTuning(name string, transponder int, downHz int64) (Sa
 		return SatTuning{}, fmt.Errorf("%s has no transponder listed", b.Name)
 	}
 	t := b.Transponders[transponder]
+	if downHz <= 0 {
+		// While the tracker is running it owns the nominal frequency — it moves
+		// as the operator tunes. Reading the centre of the passband instead would
+		// show a frequency nobody is on the moment they hunt for a station.
+		downHz = a.satTrackedNominal(b.Name, transponder)
+	}
 	if downHz <= 0 {
 		downHz = t.Centre()
 	}