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