feat(psu): switch a Modbus RTU bench supply from OpsLog

The manufacturer's document arrived, so this is no longer guesswork: 9600 8N1,
function codes 03 and 06 only, and a register map with the output on/off at
0x0001, the measurements at 0x0010…0x0013 and the set points at 0x0030/0x0031.

ONE REGISTER IS WRITTEN — 0x0001, the output. The map also exposes the voltage
and current set points and the three protection trip levels as writable, and
none of them belong to a logbook: a wrong value there is 30 V where a radio
expected 13.8, or a trip level lifted on a supply feeding an amplifier. They are
read and displayed instead, next to the measured values, which is also how an
operator sees at a glance that the supply is on and the radio is drawing nothing.

The wire layer is tested where it can be. CRC-16/MODBUS is pinned against its
published check value — the CRC of "123456789" is 0x4B37 — which fixes the
polynomial, the initial value, the reflection and the absence of a final xor all
at once; the rest of the protocol is checked frame by frame against the manual,
including the byte order of the CRC, an exception reply told apart from a broken
line, and a reply from another slave on the bus refused. The write echo must
match the value sent: it is the only confirmation the output really switched,
and accepting the frame without it is how a radio ends up dark behind a green
light.

Framing follows the manual's own rules: 3.5 character times of silence between
frames (4 ms at 9600), and a frame is over when the line falls quiet — Modbus
RTU has no terminator, and a serial read that times out returns (0, nil) here,
so the reader is built on a deadline and a quiet-time rather than on an error
that never comes.

Untested against hardware — nobody here has the supply.

frontend/wailsjs/go/models.ts

@@ -2733,6 +2733,24 @@ export namespace main {
 		}
 	}
 	
+	export class PSUSettings {
+	    enabled: boolean;
+	    com_port: string;
+	    baud: number;
+	    address: number;
+	
+	    static createFrom(source: any = {}) {
+	        return new PSUSettings(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.enabled = source["enabled"];
+	        this.com_port = source["com_port"];
+	        this.baud = source["baud"];
+	        this.address = source["address"];
+	    }
+	}
 	export class QSLBulkUpdate {
 	    sent_status: string;
 	    rcvd_status: string;
@@ -4063,6 +4081,39 @@ export namespace pskr {
 
 }
 
+export namespace psu {
+	
+	export class Status {
+	    connected: boolean;
+	    on: boolean;
+	    volts: number;
+	    amps: number;
+	    watts: number;
+	    set_volts: number;
+	    set_amps: number;
+	    protected: number;
+	    error?: string;
+	
+	    static createFrom(source: any = {}) {
+	        return new Status(source);
+	    }
+	
+	    constructor(source: any = {}) {
+	        if ('string' === typeof source) source = JSON.parse(source);
+	        this.connected = source["connected"];
+	        this.on = source["on"];
+	        this.volts = source["volts"];
+	        this.amps = source["amps"];
+	        this.watts = source["watts"];
+	        this.set_volts = source["set_volts"];
+	        this.set_amps = source["set_amps"];
+	        this.protected = source["protected"];
+	        this.error = source["error"];
+	    }
+	}
+
+}
+
 export namespace qslcard {
 	
 	export class Bevel {