fix(elecraft): calibrate the S-meter, and stop the SWR spike lingering

The S-unit mapping was provisional, waiting for a real radio. One arrived:
side by side with a K3's own display, raw 5 reads S7 and raw 9 reads S9+20,
so S9 sits near raw 6.5 — not 9 — and each raw step above it is worth ~8 dB.
The old scale showed everything two S-units low.

The SWR meter borrowed the power meter's needle inertia — hold the peak, then
close a quarter of the gap per poll. The K3 throws a brief SWR spike as an FT8
frame ends, and that decay turned one bad sample into twelve seconds of red on
the next transmission. SWR now holds the peak for the same 1.5 s and then
snaps back to the live reading: it is a warning light, not a needle.

internal/cat/kenwood_panel.go

@@ -349,7 +349,7 @@ func (k *Kenwood) readTXMeters() {
 	if v, ok := k.askNum("SW;", "SW", 3); ok {
 		k.panel.SWRRaw = v
 		if v > 0 {
-			k.panel.SWR = float64(k.swrPeak.update(v, now)) / 10
+			k.panel.SWR = float64(k.swrPeak.updateSnap(v, now)) / 10
 		}
 	}
 	if k.metersLogged >= 20 {