feat: CAT Icom working as well as split (IC7610)

internal/cat/civ/civ.go

@@ -32,6 +32,8 @@ const (
 	CmdReadMode       = 0x04
 	CmdSetFreq        = 0x05
 	CmdSetMode        = 0x06
+	CmdSplit          = 0x0F // read/set split & duplex (0x00 off, 0x01 split on)
+	CmdVfoFreq        = 0x25 // read a specific VFO's freq (sub 0x00 selected, 0x01 unselected)
 	CmdPTT            = 0x1C // sub 0x00 = PTT
 	CmdExtra          = 0x1A // sub 0x06 = data mode on modern Icoms
 	CmdReadID         = 0x19 // sub 0x00 = rig's own CI-V address (identifies model)
@@ -40,8 +42,10 @@ const (
 	CmdLevel = 0x14 // analogue levels (sub + 2 BCD bytes, 0000-0255)
 	CmdSwitch = 0x16 // on/off + multi-state DSP settings (sub + 1 byte)
 
-	SubDataMode = 0x06
-	SubPTT      = 0x00
+	SubDataMode      = 0x06
+	SubPTT           = 0x00
+	SubVfoSelected   = 0x00 // CmdVfoFreq: the active/RX VFO
+	SubVfoUnselected = 0x01 // CmdVfoFreq: the other VFO (TX in split)
 
 	// CmdLevel sub-commands.
 	SubLevelAF = 0x01 // AF (volume)