fix: CI-V address 00, simplex uploaded as split, and S/F spots

Three field reports.

Icom CI-V address 00 could not be kept: zero was read as "not
configured" in all three places that validate it, so every save put the
rig back to the IC-7610's 0x98 — and the model dropdown followed, since
it is derived from the address rather than stored. Picking "Other
(custom address)" also had no effect of its own: the list re-derived
itself and snapped back to whatever rig matched. It now stays chosen.

Cloudlog/Wavelog showed "17m/17m" on ordinary FT8 contacts (OE6CLD).
Every QSO is stamped with a receive side equal to the transmit side, and
the uploaded record carried it; Wavelog draws band/band_rx whenever both
are there. In ADIF an absent BAND_RX means "same as transmit", so the
uploaded record now writes the receive side only when it differs. The
copy forwarded to another logger over UDP keeps writing it in full —
that is why it was stamped in the first place (Log4OM reads BAND_RX) —
through its own ForwardRecordADIF.

"S/F" in a spot comment joins superfox / sfox / F-H as FT8.

internal/cat/icomnet.go

@@ -43,7 +43,7 @@ var icnBE = binary.BigEndian
 // = CI-V only (the proven default). The audio stream is fully separate from CAT,
 // so enabling it can't affect freq/mode/DSP control.
 func NewIcomNet(host, user, pass string, civAddr int, digitalDefault string, audioSink func([]byte)) *IcomSerial {
-	if civAddr <= 0 || civAddr > 0xFF {
+	if civAddr < 0 || civAddr > 0xFF {
 		civAddr = 0x98 // IC-7610
 	}
 	if digitalDefault == "" {

internal/cat/icomserial.go

@@ -172,12 +172,13 @@ const (
 )
 
 // NewIcomSerial builds an (unconnected) Icom serial backend. baud defaults to
-// 115200, rig address to the IC-7610's 0x98 when out of range.
+// 115200, rig address to the IC-7610's 0x98 when out of range — and 0x00 is in
+// range: it is a valid CI-V address that some rigs and interfaces are set to.
 func NewIcomSerial(portName string, baud, civAddr int, digitalDefault string) *IcomSerial {
 	if baud <= 0 {
 		baud = 115200
 	}
-	if civAddr <= 0 || civAddr > 0xFF {
+	if civAddr < 0 || civAddr > 0xFF {
 		civAddr = 0x98 // IC-7610
 	}
 	if digitalDefault == "" {