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_test.go

@@ -123,7 +123,7 @@ func TestYaesuModeDigit(t *testing.T) {
 		{"", 14074000, 0},      // nothing to set
 	}
 	for _, c := range cases {
-		if got := yaesuModeDigit(c.mode, c.hz); got != c.want {
+		if got := yaesuModeDigit(c.mode, c.hz, false); got != c.want {
 			t.Errorf("yaesuModeDigit(%q, %d) = %q, want %q", c.mode, c.hz, got, c.want)
 		}
 	}
@@ -406,3 +406,22 @@ func TestYaesuAntennaCommand(t *testing.T) {
 		}
 	}
 }
+
+// ADIF says "RTTY" and stops there, but Yaesu has both sidebands and the rig
+// has to be told one. LSB is the older convention and the default; the other is
+// a station's own choice, made once.
+func TestYaesuRTTYSideband(t *testing.T) {
+	if got := yaesuModeDigit("RTTY", 14_080_000, false); got != '6' {
+		t.Errorf("RTTY = %q, want RTTY-L", got)
+	}
+	if got := yaesuModeDigit("RTTY", 14_080_000, true); got != '9' {
+		t.Errorf("RTTY (upper) = %q, want RTTY-U", got)
+	}
+	// The switch is about RTTY and nothing else.
+	if got := yaesuModeDigit("FT8", 28_074_000, true); got != 'C' {
+		t.Errorf("FT8 = %q, want DATA-U", got)
+	}
+	if got := yaesuModeDigit("CW", 14_030_000, true); got != '3' {
+		t.Errorf("CW = %q, want CW-U", got)
+	}
+}