feat(icom): PTT and offset split at the operator's hand

The MOX and SPLIT controls lived at the bottom of the Transmit card, below
the fold on most layouts — a console whose PTT needs scrolling is not a
console. A strip under the mode row now carries the PTT and Split as
OFF/+1k/+5k/+10k, DXpedition style, on a new SetIcomSplitOffset that takes
the chosen TX offset (0 keeps the old CW/SSB convention). The mode-alignment
from the previous fix rides along on every engage.

internal/cat/cat.go

@@ -640,6 +640,7 @@ type IcomController interface {
 	SetRFPower(int) error
 	SetMicGain(int) error
 	SetIcomSplit(bool) error
+	SetIcomSplitOffset(bool, int64) 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

@@ -1860,10 +1860,16 @@ func (b *IcomSerial) SetMicGain(p int) error {
 }
 
 func (b *IcomSerial) SetIcomSplit(on bool) error {
+	return b.SetIcomSplitOffset(on, 0)
+}
+
+// SetIcomSplitOffset enables split with a CHOSEN TX offset in Hz (0 = the
+// usual convention: +1 kHz on CW, +5 kHz otherwise). The console offers
+// +1/+5/+10 directly, DXpedition style.
+func (b *IcomSerial) SetIcomSplitOffset(on bool, offsetHz int64) error {
 	if on {
-		// Enable split with the usual "work him up" TX offset: +1 kHz on CW,
-		// +5 kHz otherwise (SSB). Set the unselected (TX) VFO to RX+offset first,
-		// then turn split on. 0x25 0x01 + BCD sets the unselected VFO's frequency.
+		// Set the unselected (TX) VFO to RX+offset first, then turn split on.
+		// 0x25 0x01 + BCD sets the unselected VFO's frequency.
 		rx := b.curFreq
 		if rx <= 0 {
 			if hz, err := b.readFreq(); err == nil {
@@ -1871,9 +1877,12 @@ func (b *IcomSerial) SetIcomSplit(on bool) error {
 			}
 		}
 		if rx > 0 {
-			offset := int64(5000)
-			if b.curModeByte == civ.ModeCW || b.curModeByte == civ.ModeCWR {
-				offset = 1000
+			offset := offsetHz
+			if offset <= 0 {
+				offset = 5000
+				if b.curModeByte == civ.ModeCW || b.curModeByte == civ.ModeCWR {
+					offset = 1000
+				}
 			}
 			_ = b.exec(append([]byte{civ.CmdVfoFreq, civ.SubVfoUnselected}, civ.FreqToBCD(rx+offset)...)...)
 		}