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/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)