feat(elecraft): a K3/K4 console — power, volume, S-meter, SWR, MOX, tune

The Elecraft backend already existed; what was missing was somewhere to
operate the radio from. This adds the panel, on the Kenwood-dialect
client the K3 already speaks, in a tab of its own beside the Yaesu and
Icom consoles.

Scope is the six controls asked for and nothing else. Every extra command
added without a radio to test it against is a control that may or may not
do what its label says, and a K3 exposes dozens.

No K3 was available while writing this, so the two halves are treated
differently. The setters are the commands the reference documents
unambiguously and whose effect is visible and reversible (PC, AG,
TX/RX). The meters are the opposite: their scaling differs by model and
firmware, so the raw answers are logged next to the power setting, the
panel says the scaling is provisional, and an unmeasured SWR shows as
'—' rather than as a perfect 1.0 — a good match on an antenna nobody
measured is the reading that costs a radio.

ATU tune sends SWT20 (the K3 front-panel tap) and logs exactly what it
sent, so a wrong mapping names itself instead of leaving an operator
guessing which button OpsLog pressed.

internal/cat/cat.go

@@ -990,6 +990,30 @@ type KenwoodController interface {
 	SetKeySpeed(int) error
 }
 
+// KenwoodState returns the K3/K4 panel snapshot, or (zero, false) when the
+// active backend is not a Kenwood-dialect rig.
+func (m *Manager) KenwoodState() (KenwoodTXState, bool) {
+	m.mu.RLock()
+	b := m.backend
+	m.mu.RUnlock()
+	if kc, ok := b.(KenwoodPanelController); ok {
+		return kc.KenwoodState(), true
+	}
+	return KenwoodTXState{}, false
+}
+
+// KenwoodPanelDo dispatches a K3/K4 panel control onto the CAT goroutine, so a
+// panel click and the poll loop never share the serial port at the same instant.
+func (m *Manager) KenwoodPanelDo(fn func(KenwoodPanelController) error) error {
+	return m.exec(func(b Backend) error {
+		kc, ok := b.(KenwoodPanelController)
+		if !ok {
+			return fmt.Errorf("active CAT backend is not a Kenwood/Elecraft")
+		}
+		return fn(kc)
+	})
+}
+
 // KenwoodDo dispatches a Kenwood control onto the CAT goroutine.
 func (m *Manager) KenwoodDo(fn func(KenwoodController) error) error {
 	return m.exec(func(b Backend) error {