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/yaesu.go

@@ -95,6 +95,11 @@ type Yaesu struct {
 
 	curFreq int64
 	curVFO  string // "A" or "B"
+
+	// Control-panel state and its slow-beat counter — see yaesu_panel.go.
+	panel       YaesuTXState
+	panelCycle  int
+	panelLoaded bool
 }
 
 func NewYaesu(portName string, baud int, digital string) *Yaesu {
@@ -216,6 +221,7 @@ func (y *Yaesu) ReadState() (RigState, error) {
 			debugLog.Printf("yaesu: unknown mode reply %q", r)
 		}
 	}
+	y.readPanel(s.Mode, s.Split)
 	return s, nil
 }