fix: hold the TX meters through the gaps between words

A quarter-per-poll decay covers the milliseconds between CW elements but not the
gaps that matter on the air: between the words of a CQ, in CW as in SSB, the
meters genuinely read 0 for most of a second and the bars fell with them.

A peak now stands for 1.5 s before it starts to fall, and still rises instantly —
a needle goes up fast and comes down slow. The SWR RATIO is not updated at all
from a zero reading: showing 1.0 during a word gap is worse than showing a stale
figure, because it looks like good news.

A test caught a real defect on the way: the proportional decay stalls on
integers. With the needle at 13 and the truth at 10, a quarter of the gap rounds
to zero and the meter sat three units high for ever. It now always steps down by
at least one, so it converges.

internal/cat/yaesu.go

@@ -104,6 +104,10 @@ type Yaesu struct {
 	panelLoaded bool
 	// Commands this rig answered "?;" to — asked once, then never again.
 	unsupported map[string]bool
+	// Needle inertia for the TX meters — see meterPeak.
+	powerPeak  meterPeak
+	powerWPeak meterPeak
+	swrPeak    meterPeak
 	// metersLogged counts the RM1..RM6 samples taken during transmission, so the
 	// survey follows a real carrier instead of catching one instant of it.
 	metersLogged int