fix(icom): the SUB dial is always shown, and split aligns the TX mode

The unselected VFO was read only when split was on, so a dual-receiver rig's
second dial sat blank on the console. It is now read on the same slow beat
regardless. And engaging split copies the main's mode (and data flag) onto
the TX VFO with 0x26 01 — a split whose TX VFO still speaks yesterday's mode
transmits FM into a CW pileup; rigs that predate 0x26 NAK it harmlessly.

internal/cat/cat.go

@@ -572,9 +572,13 @@ type IcomTXState struct {
 	// Transmit + live status (polled).
 	Transmitting bool `json:"transmitting"`
 	Split        bool `json:"split"`
-	SMeter       int  `json:"s_meter"`     // 0-100 (raw 0-255; S9≈120)
-	PowerMeter   int  `json:"power_meter"` // 0-100 (TX Po)
-	SWRMeter     int  `json:"swr_meter"`   // 0-100 (TX SWR)
+	// SubHz is the unselected VFO / sub receiver's frequency — shown on the
+	// console's SUB display whether or not split is on: a dual-receiver rig
+	// (IC-7610/7760) has a second dial worth seeing at all times.
+	SubHz      int64 `json:"sub_hz"`
+	SMeter     int   `json:"s_meter"`     // 0-100 (raw 0-255; S9≈120)
+	PowerMeter int   `json:"power_meter"` // 0-100 (TX Po)
+	SWRMeter   int   `json:"swr_meter"`   // 0-100 (TX SWR)
 	// RIT / ΔTX (XIT).
 	RITHz int  `json:"rit_hz"` // RIT/XIT offset, signed Hz
 	RITOn bool `json:"rit_on"`