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/models.ts

@@ -1442,6 +1442,10 @@ export namespace main {
 	    port: number;
 	    com_port: string;
 	    baud: number;
+	    freq_out: boolean;
+	    freq_com_port: string;
+	    freq_baud: number;
+	    freq_broadcast_ms: number;
 	
 	    static createFrom(source: any = {}) {
 	        return new AmpConfig(source);
@@ -1458,6 +1462,10 @@ export namespace main {
 	        this.port = source["port"];
 	        this.com_port = source["com_port"];
 	        this.baud = source["baud"];
+	        this.freq_out = source["freq_out"];
+	        this.freq_com_port = source["freq_com_port"];
+	        this.freq_baud = source["freq_baud"];
+	        this.freq_broadcast_ms = source["freq_broadcast_ms"];
 	    }
 	}
 	export class AmpStatus {