feat(icom): the ATU can be taken out of line

TUNE (1C 01 02) started a cycle; nothing sent the 0 that puts the tuner back
through. An ATU chip beside SPLIT now toggles it, and the tuner state is read
on the slow front-panel beat so the radio's own TUNER button stays in sync.

internal/cat/cat.go

@@ -602,7 +602,8 @@ type IcomTXState struct {
 	Att     int    `json:"att"`           // dB attenuation, 0=off
 	Filter  int    `json:"filter"`        // 1 | 2 | 3 (FIL1/2/3)
 	// Antenna (IC-7610 = ANT1/ANT2).
-	Antenna int `json:"antenna"` // 1 | 2 (0 = unknown)
+	Antenna int  `json:"antenna"` // 1 | 2 (0 = unknown)
+	ATUOn   bool `json:"atu_on"`  // internal tuner engaged
 	// Filter fine controls: Twin PBT + manual notch (0-100, 50 = centre).
 	PBTInner    int  `json:"pbt_inner"`
 	PBTOuter    int  `json:"pbt_outer"`
@@ -641,6 +642,7 @@ type IcomController interface {
 	SetMicGain(int) error
 	SetIcomSplit(bool) error
 	SetIcomSplitOffset(bool, int64) error
+	SetATU(bool) error
 	TuneATU() error
 	SetScope(bool) error              // enable/disable the spectrum-scope waveform stream
 	SetScopeMode(bool) error          // true = fixed span, false = center-on-VFO

internal/cat/icomserial.go

@@ -530,6 +530,13 @@ func (b *IcomSerial) refreshFrontPanel() {
 			b.dsp.Filter = int(f)
 			b.dspMu.Unlock()
 		}
+	case 2:
+		// Tuner in/out — the radio's own TUNER button changes it mid-session.
+		if v, ok := b.readSwitchSub(civ.CmdATU, civ.SubATU); ok {
+			b.dspMu.Lock()
+			b.dsp.ATUOn = v == 1
+			b.dspMu.Unlock()
+		}
 	}
 }
 
@@ -2041,6 +2048,17 @@ func (b *IcomSerial) TuneATU() error {
 	return b.exec(civ.CmdATU, civ.SubATU, 0x02)
 }
 
+// SetATU engages or DISENGAGES the internal tuner (0x1C 0x01: 1 = in line,
+// 0 = through). Tune existed without this, which meant a tuner once engaged
+// could not be taken out of line again from the console.
+func (b *IcomSerial) SetATU(on bool) error {
+	if err := b.exec(civ.CmdATU, civ.SubATU, boolByte(on)); err != nil {
+		return err
+	}
+	b.setCache(func(s *IcomTXState) { s.ATUOn = on })
+	return nil
+}
+
 func (b *IcomSerial) setCache(fn func(*IcomTXState)) {
 	b.dspMu.Lock()
 	fn(&b.dsp)