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.

internal/cat/cat.go

@@ -832,6 +832,50 @@ func (m *Manager) IcomDo(fn func(IcomController) error) error {
 	})
 }
 
+// SatTuner is a backend that can be put on a satellite: a receiver on one band
+// and a transmitter on another, both moving under Doppler, at the same time.
+//
+// It is a separate interface from the per-manufacturer ones because what a
+// satellite needs is not a manufacturer's feature — it is a shape of operating
+// that a FlexRadio and an IC-9700 both provide and reach in completely
+// different ways. A backend that cannot do it simply does not implement this,
+// and the caller falls back to tuning the downlink alone rather than pretending.
+type SatTuner interface {
+	// SetSatellite arms or disarms satellite operation: the rig's own satellite
+	// mode where it has one, two slices where it has those. Disarming must leave
+	// the radio somewhere an operator can work from, not half-configured.
+	SetSatellite(on bool) error
+	// TuneSatellite points the receiver at downHz and the transmitter at upHz,
+	// both already Doppler-corrected. Modes are ADIF names ("SSB", "FM", "CW");
+	// an empty one leaves that side's mode alone.
+	TuneSatellite(downHz, upHz int64, downMode, upMode string) error
+	// SatReceiveHz is where the receiver actually is. The operator tunes it to
+	// follow a station across a linear transponder, and that dial movement is
+	// the input the whole tracker works from — without reading it back, a
+	// tracker fights the operator instead of helping them.
+	SatReceiveHz() (int64, error)
+}
+
+// SatCapable reports whether the active backend can hold a satellite pair.
+func (m *Manager) SatCapable() bool {
+	m.mu.RLock()
+	b := m.backend
+	m.mu.RUnlock()
+	_, ok := b.(SatTuner)
+	return ok
+}
+
+// SatDo dispatches a satellite control onto the CAT goroutine.
+func (m *Manager) SatDo(fn func(SatTuner) error) error {
+	return m.exec(func(b Backend) error {
+		st, ok := b.(SatTuner)
+		if !ok {
+			return fmt.Errorf("this radio cannot hold a satellite pair from OpsLog")
+		}
+		return fn(st)
+	})
+}
+
 // exec marshals a backend operation onto the CAT goroutine. Returns the
 // operation's error or a "busy"/"not running" error if dispatch failed.
 func (m *Manager) exec(fn func(Backend) error) error {