feat: added RIT and XIT for Icom

internal/cat/civ/civ.go

@@ -44,6 +44,11 @@ const (
 	CmdSwitch = 0x16 // on/off + multi-state DSP settings (sub + 1 byte)
 	CmdATU    = 0x1C // sub 0x01 = antenna tuner (0x00 off, 0x01 through, 0x02 tune)
 	CmdScope  = 0x27 // spectrum-scope waveform stream (sub 0x00 = data, 0x11 = on/off)
+	CmdRIT    = 0x21 // RIT/ΔTX: sub 0x00 offset freq, 0x01 RIT on/off, 0x02 ΔTX(XIT) on/off
+
+	SubRITFreq = 0x00 // RIT/ΔTX offset: 2 BCD bytes (LE, 0-9999) + sign byte (00 +, 01 -)
+	SubRITOn   = 0x01 // RIT on/off (00/01)
+	SubXITOn   = 0x02 // ΔTX (XIT) on/off (00/01)
 
 	SubDataMode      = 0x06
 	SubPTT           = 0x00
@@ -154,6 +159,38 @@ func BCDToLevel(b []byte) int {
 	return int(b[0])*100 + int(b[1]>>4)*10 + int(b[1]&0x0F)
 }
 
+// RITToBCD encodes a RIT/ΔTX offset (Hz, −9999..9999) as the 3 bytes CI-V
+// command 0x21 0x00 uses: 2 little-endian BCD bytes of the magnitude followed by
+// a sign byte (0x00 positive, 0x01 negative).
+func RITToBCD(hz int) []byte {
+	neg := hz < 0
+	if neg {
+		hz = -hz
+	}
+	if hz > 9999 {
+		hz = 9999
+	}
+	lo := byte(hz%10 | (hz/10%10)<<4)
+	hi := byte(hz/100%10 | (hz/1000%10)<<4)
+	sign := byte(0)
+	if neg {
+		sign = 1
+	}
+	return []byte{lo, hi, sign}
+}
+
+// BCDToRIT decodes the 3 offset bytes of a 0x21 0x00 response back to signed Hz.
+func BCDToRIT(b []byte) int {
+	if len(b) < 3 {
+		return 0
+	}
+	v := int(b[0]&0x0F) + int(b[0]>>4)*10 + int(b[1]&0x0F)*100 + int(b[1]>>4)*1000
+	if b[2] != 0 {
+		return -v
+	}
+	return v
+}
+
 // ByteToBCD / BCDToByte handle a single packed-BCD byte (used by the
 // attenuator, where the value is dB: 0x00, 0x06, 0x12, 0x18…).
 func ByteToBCD(v int) byte {