fix: the Yaesu power meter is not linear — calibrate it on three points

One point could not reveal the curve. Scaling 207 = 100 W straight down read
30 W where the radio showed 10, and 75 where it showed 50 — wrong everywhere
except at the single point it was fitted to.

Three readings taken against the rig's own display give the shape:

  raw  62 → 10 W
  raw 155 → 50 W
  raw 207 → 100 W

Interpolating between them reproduces the radio exactly at those points and stays
close in between. I did not fit a formula: three samples can be made to support
several curves, and the operator can check a table against their own meter.

Above the top the last segment's slope continues rather than clamping, so a rig
driving an amplifier does not sit pinned at 100 W. A test pins the measured pairs
and the monotonicity, so a later change that breaks them fails against the radio
rather than against taste.

internal/cat/yaesu_panel.go

@@ -713,20 +713,52 @@ func swrFromReflection(raw int) float64 {
 
 // yaesuWatts converts the power-meter reading to watts.
 //
-// Single-point calibration, measured rather than assumed: an FTDX10 keying CW at
-// 100 W reads 207. The PO meter is close enough to linear in power for one point
-// to carry the scale, and a figure taken from the METER beats the power SETTING,
-// which says what was asked for and not what left the radio.
+// The meter is NOT linear in power, which one calibration point could never
+// reveal: scaling 207 = 100 W straight down read 30 W where the rig showed 10,
+// and 75 where it showed 50. Three points measured against the radio's own
+// display settle the curve:
 //
-// A second point at low power would refine it; until then the number is right at
-// 100 W and approximate below.
-const yaesuPowerFullScale = 207.0 // raw reading measured at 100 W
+//	raw  62 → 10 W
+//	raw 155 → 50 W
+//	raw 207 → 100 W
+//
+// Interpolating between them reproduces the rig exactly at those points and
+// stays close in between — better than fitting a formula to three samples and
+// pretending it holds everywhere. Above the last point it keeps the final
+// segment's slope, so an amplifier-driving rig does not flatten at 100 W.
+//
+// Measured on an FTDX10 (2026-07-29). Another model may well need its own row.
+var yaesuPowerCurve = []struct {
+	raw   int
+	watts float64
+}{
+	{0, 0},
+	{62, 10},
+	{155, 50},
+	{207, 100},
+}
 
 func yaesuWatts(raw int) float64 {
 	if raw <= 0 {
 		return 0
 	}
-	return float64(raw) * 100.0 / yaesuPowerFullScale
+	for i := 1; i < len(yaesuPowerCurve); i++ {
+		hi := yaesuPowerCurve[i]
+		if raw <= hi.raw {
+			lo := yaesuPowerCurve[i-1]
+			span := float64(hi.raw - lo.raw)
+			if span <= 0 {
+				return hi.watts
+			}
+			f := float64(raw-lo.raw) / span
+			return lo.watts + f*(hi.watts-lo.watts)
+		}
+	}
+	// Past the top of the curve: extend the last segment rather than clamp.
+	n := len(yaesuPowerCurve)
+	lo, hi := yaesuPowerCurve[n-2], yaesuPowerCurve[n-1]
+	slope := (hi.watts - lo.watts) / float64(hi.raw-lo.raw)
+	return hi.watts + float64(raw-hi.raw)*slope
 }
 
 // meterPeak gives a meter the inertia a needle has: it jumps to a higher reading
@@ -735,8 +767,7 @@ func yaesuWatts(raw int) float64 {
 // The hold is the part that matters on the air. Between CW elements the gap is
 // milliseconds, but between the words of a CQ — in CW as in SSB — it is most of
 // a second, and a meter that decays straight away reads 0 in every one of those
-// gaps. The operator sees a bar flashing rather than the power and SWR they are
-// actually running.
+// gaps: the operator sees a bar flashing rather than the power they are running.
 type meterPeak struct {
 	val int
 	at  time.Time
@@ -757,7 +788,7 @@ func (m *meterPeak) update(sample int, now time.Time) int {
 		return m.val // still inside the hold — the needle has not started to fall
 	}
 	// At least one step down, always. A proportional decay on integers stalls
-	// near the end — with the needle at 13 and the truth at 10, a quarter of the
+	// near the end: with the needle at 13 and the truth at 10, a quarter of the
 	// gap rounds to zero and the meter sits three units high for ever.
 	step := (m.val - sample) / meterDecay
 	if step < 1 {