feat: amplifier band-follow — answer the amp's frequency polls

On its CAT/AUX connector an ACOM is the MASTER: it polls a transceiver and
changes band from the reply, so nothing can be pushed to it. internal/catemu
answers those polls on a second serial port, in ACOM command set 5 (Kenwood /
Elecraft): FA;, FB;, IF; and ID;, and nothing else — a wrong-length reply is
worse than none, it desynchronises the amp's parser for the following poll.

Also offered for SPE. Some amps do not poll at all but read the radio↔PC CAT
line in parallel; those never hear a responder, so an optional unprompted send
(500/1000 ms) covers them.

The TX frequency is what is sent: in split the amp must be tuned where we
transmit. Frame lengths are pinned by tests — the failure they guard against is
silent and only shows up as an amp that mistunes.

Untested on hardware.

frontend/wailsjs/go/main/App.d.ts

@@ -6,6 +6,7 @@ import {main} from '../models';
 import {cat} from '../models';
 import {profile} from '../models';
 import {acom} from '../models';
+import {catemu} from '../models';
 import {antgenius} from '../models';
 import {award} from '../models';
 import {awardref} from '../models';
@@ -344,6 +345,8 @@ export function GetActiveProfile():Promise<profile.Profile>;
 
 export function GetAlertEmailTo():Promise<string>;
 
+export function GetAmpBandFollowStatus():Promise<Record<string, catemu.Status>>;
+
 export function GetAmpStatuses():Promise<Array<main.AmpStatus>>;
 
 export function GetAmplifiers():Promise<Array<main.AmpConfig>>;

frontend/wailsjs/go/main/App.js

@@ -638,6 +638,10 @@ export function GetAlertEmailTo() {
   return window['go']['main']['App']['GetAlertEmailTo']();
 }
 
+export function GetAmpBandFollowStatus() {
+  return window['go']['main']['App']['GetAmpBandFollowStatus']();
+}
+
 export function GetAmpStatuses() {
   return window['go']['main']['App']['GetAmpStatuses']();
 }