feat: Yaesu control panel (meters, bands, DSP, TX), and drop a stale Flex hint

A console pane for the native Yaesu backend, in the same shape as the Icom and
Flex ones: S/PO/SWR meters, band and mode rows, AF/RF/squelch, AGC, the IPO/AMP1/
AMP2 front-end selector, ATT, NB, DNR + level, narrow filter, power in watts, mic
gain, VOX, split and ATU tune.

Three decisions worth keeping:

Panel reads are STAGGERED and live in their own file, away from ReadState. Meters
poll every cycle; settings only change when someone turns a knob, so they refresh
every 8th cycle and right after any set. Polling all of it every cycle would put
twenty queries a second on the serial link the frequency display shares.

Band buttons use the rig's own band memory (BS) rather than a frequency we pick,
so 20 m lands where the operator last was on 20 m — what the radio's own band
keys do.

A set is followed by a read-back, so the panel shows what the RIG ended up with,
not what we asked for; the two differ whenever a value is out of range or the
mode forbids the control. And a control the model lacks keeps its previous value
instead of dropping to zero, which reads as a setting that reset itself.

The S9 point of the S-meter scale is a hypothesis (the manual does not state it)
and is commented as such — one number to correct if reports come out an S unit
off, rather than a fudge spread through the RST helper.

Also removes the FlexRadio settings blurb, which explained the backend to
someone who had already chosen it.

internal/cat/cat.go

@@ -845,3 +845,27 @@ func BandFromHz(hz int64) string {
 	}
 	return ""
 }
+
+// YaesuState returns the panel snapshot, or (zero, false) when the active
+// backend isn't a Yaesu — same shape as IcomState.
+func (m *Manager) YaesuState() (YaesuTXState, bool) {
+	m.mu.RLock()
+	b := m.backend
+	m.mu.RUnlock()
+	if yc, ok := b.(YaesuController); ok {
+		return yc.YaesuState(), true
+	}
+	return YaesuTXState{}, false
+}
+
+// YaesuDo dispatches a Yaesu control onto the CAT goroutine, so a panel click
+// and the poll loop never share the serial port at the same instant.
+func (m *Manager) YaesuDo(fn func(YaesuController) error) error {
+	return m.exec(func(b Backend) error {
+		yc, ok := b.(YaesuController)
+		if !ok {
+			return fmt.Errorf("active CAT backend is not a Yaesu")
+		}
+		return fn(yc)
+	})
+}