fix: IC-7300 (single-scope Icom) blank spectrum scope in center/VFO mode

The multi-frame waveform header (USB path) read the second frequency field
as an absolute high edge, but in center mode it is a span — so high < low and
the panadapter had no valid range to map the trace onto (blank scope). It now
applies the same center+span vs low+high disambiguation the IC-7610 single-
frame path already used. FIXED mode was unaffected.

Also stamps changelog 0.20.10 with today's date.

internal/cat/icomserial.go

@@ -661,9 +661,23 @@ func (b *IcomSerial) scopeLoop(spec chan civ.Decoded, done chan struct{}) {
 			if seq == 1 { // header frame — begins a new sweep, no waveform data
 				regions = make(map[byte][]byte)
 				total = tot
-				if len(region) >= 11 { // [info][low 5][high 5]
-					low := civ.BCDToFreq(region[1:6])
-					high := civ.BCDToFreq(region[6:11])
+				if len(region) >= 11 { // [info][freq1 5-BCD][freq2 5-BCD]
+					// Same center/fixed disambiguation as the single-frame (net) path:
+					// FIXED sends [low-edge][high-edge] (both absolute), CENTER sends
+					// [center][span]. Tell them apart by magnitude — a second value
+					// BIGGER than the first is a real high edge; a smaller one is a
+					// span. Without this, an IC-7300 in center (VFO-following) mode set
+					// high = span (e.g. 100 kHz), so high < low and the panadapter had
+					// no valid range to map the trace onto → blank scope. FIXED mode
+					// parsed fine, which is why only center mode was broken.
+					v1 := civ.BCDToFreq(region[1:6])
+					v2 := civ.BCDToFreq(region[6:11])
+					var low, high int64
+					if v2 > v1 {
+						low, high = v1, v2 // absolute low/high edges (fixed)
+					} else {
+						low, high = v1-v2/2, v1+v2/2 // centre + span (centre-on-VFO)
+					}
 					b.scopeMu.Lock()
 					b.scopeLow, b.scopeHigh = low, high
 					b.scopeMu.Unlock()