feat: a typed watering hole carries its mode; Yaesu RTTY sideband

His log settles the 28.074 case: "SetCATFrequency 28.074 MHz" and the
state still reads mode=USB — nothing sent a mode, so the rig simply
stayed where it was. A spot click has always carried one; a frequency
typed by hand carried none. It now uses the same table and the same
tolerance as a spot (±3 kHz of a known FT8/FT4/JS8 frequency), and only
towards the digital modes: tuning AWAY from one leaves the mode alone,
because there the frequency says nothing about what the operator means.

RTTY on Yaesu is a choice the log cannot make: ADIF records "RTTY" and
the rig has MD06 (RTTY-L) and MD09 (RTTY-U). The older lower sideband
stays the default and a station whose FSK controller wants the upper one
says so once in Settings → CAT.

internal/cat/yaesu.go

@@ -108,8 +108,10 @@ type Yaesu struct {
 	// and VS is used — see ReadState.
 	rxVFOCmd string
 	// freqDigits is how many digits this rig writes a frequency in, LEARNED from
-	// its own replies. See setFreqWidth.
+	// its own replies. See learnFreqWidth.
 	freqDigits int
+	// rttyUpper picks RTTY-U over RTTY-L — see SetRTTYUpper.
+	rttyUpper bool
 
 	curFreq   int64
 	curRXFreq int64
@@ -140,6 +142,18 @@ func NewYaesu(portName string, baud int, digital string) *Yaesu {
 	return &Yaesu{portName: strings.TrimSpace(portName), baud: baud, digital: digital, curVFO: "A"}
 }
 
+// SetRTTYUpper chooses which sideband RTTY is set on.
+//
+// Yaesu has both — MD06 is RTTY-L, MD09 is RTTY-U — and ADIF has neither: it
+// says "RTTY" and stops there, so the rig cannot be driven from the logged mode
+// alone. LSB is the older convention and stays the default; an operator whose
+// FSK controller or decoder wants the other one says so once here.
+func (y *Yaesu) SetRTTYUpper(v bool) {
+	y.mu.Lock()
+	defer y.mu.Unlock()
+	y.rttyUpper = v
+}
+
 // SetLowerLines chooses whether DTR and RTS are deasserted on connect. Set
 // before Connect.
 func (y *Yaesu) SetLowerLines(v bool) {
@@ -375,7 +389,7 @@ func (y *Yaesu) SetMode(mode string) error {
 	if y.port == nil {
 		return fmt.Errorf("yaesu: not connected")
 	}
-	d := yaesuModeDigit(mode, y.curFreq)
+	d := yaesuModeDigit(mode, y.curFreq, y.rttyUpper)
 	if d == 0 {
 		return fmt.Errorf("yaesu: no CAT mode for %q", mode)
 	}
@@ -608,7 +622,7 @@ func resolveYaesuVFOs(freqA, freqB int64, vfo string, split bool) (tx, rx int64,
 // yaesuModeDigit maps an ADIF mode to the MD digit. SSB has no single digit —
 // the sideband follows the worldwide convention (LSB below 10 MHz, USB above),
 // which is why the current frequency is part of the decision.
-func yaesuModeDigit(mode string, freqHz int64) byte {
+func yaesuModeDigit(mode string, freqHz int64, rttyUpper bool) byte {
 	switch strings.ToUpper(strings.TrimSpace(mode)) {
 	case "SSB":
 		if freqHz > 0 && freqHz < 10_000_000 {
@@ -626,7 +640,10 @@ func yaesuModeDigit(mode string, freqHz int64) byte {
 	case "AM":
 		return '5'
 	case "RTTY":
-		return '6'
+		if rttyUpper {
+			return '9' // RTTY-U
+		}
+		return '6' // RTTY-L, the older convention
 	case "":
 		return 0
 	default: