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.

app_psu.go

@@ -0,0 +1,128 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"hamlog/internal/applog"
+	"hamlog/internal/psu"
+)
+
+// ── Bench power supply (Modbus RTU) ──────────────────────────────────────────
+//
+// A programmable supply feeding the shack, switched on and off from OpsLog so
+// the station comes up and goes down with the logbook rather than by reaching
+// behind the desk.
+//
+// OpsLog READS the supply's measurements and its set points, and WRITES exactly
+// one thing: the output on/off. The register map has the voltage and current
+// set points and the three protection trip levels as writable too, and none of
+// them belong to a logbook — a wrong value there is 30 V where a radio expected
+// 13.8. See internal/psu for the map and where it comes from.
+
+const (
+	keyPSUEnabled = "psu.enabled"
+	keyPSUPort    = "psu.com_port"
+	keyPSUBaud    = "psu.baud"
+	keyPSUAddress = "psu.address" // Modbus slave address, 1…15 on this family
+)
+
+// PSUSettings is the JSON shape for the Hardware → Power supply panel.
+type PSUSettings struct {
+	Enabled bool   `json:"enabled"`
+	ComPort string `json:"com_port"`
+	Baud    int    `json:"baud"`    // 9600 from the factory
+	Address int    `json:"address"` // 1 from the factory
+}
+
+// GetPSUSettings returns the persisted supply config.
+func (a *App) GetPSUSettings() (PSUSettings, error) {
+	out := PSUSettings{Baud: 9600, Address: 1}
+	if a.settings == nil {
+		return out, fmt.Errorf("db not initialized")
+	}
+	m, err := a.settings.GetMany(a.ctx, keyPSUEnabled, keyPSUPort, keyPSUBaud, keyPSUAddress)
+	if err != nil {
+		return out, err
+	}
+	out.Enabled = m[keyPSUEnabled] == "1"
+	out.ComPort = m[keyPSUPort]
+	if v, e := strconv.Atoi(m[keyPSUBaud]); e == nil && v > 0 {
+		out.Baud = v
+	}
+	if v, e := strconv.Atoi(m[keyPSUAddress]); e == nil && v >= 1 && v <= 250 {
+		out.Address = v
+	}
+	return out, nil
+}
+
+// SavePSUSettings persists the config and (re)starts or stops the client.
+func (a *App) SavePSUSettings(s PSUSettings) error {
+	if a.settings == nil {
+		return fmt.Errorf("db not initialized")
+	}
+	if s.Baud <= 0 {
+		s.Baud = 9600
+	}
+	// The manual gives 1…15 for the address field and 1…250 for the address
+	// SETTING register. Clamped to the wider range and defaulted to 1: an
+	// address of 0 is the Modbus broadcast, which never answers, so accepting it
+	// would give a supply that is present and permanently "not responding".
+	if s.Address < 1 || s.Address > 250 {
+		s.Address = 1
+	}
+	for k, v := range map[string]string{
+		keyPSUEnabled: boolStr(s.Enabled),
+		keyPSUPort:    strings.TrimSpace(s.ComPort),
+		keyPSUBaud:    strconv.Itoa(s.Baud),
+		keyPSUAddress: strconv.Itoa(s.Address),
+	} {
+		if err := a.settings.Set(a.ctx, k, v); err != nil {
+			return err
+		}
+	}
+	a.restartAsync("psu", a.startPSU)
+	return nil
+}
+
+// startPSU stops any running client and starts a fresh one if the supply is
+// enabled and has a port. Safe to call repeatedly (startup, settings save,
+// profile switch).
+func (a *App) startPSU() {
+	if a.psu != nil {
+		go a.psu.Stop()
+		a.psu = nil
+	}
+	s, err := a.GetPSUSettings()
+	if err != nil {
+		applog.Printf("psu: not started — settings unavailable: %v", err)
+		return
+	}
+	if !s.Enabled {
+		return
+	}
+	if strings.TrimSpace(s.ComPort) == "" {
+		applog.Printf("psu: not started — no serial port configured")
+		return
+	}
+	applog.Printf("psu: starting on %s @ %d baud, Modbus address %d", s.ComPort, s.Baud, s.Address)
+	a.psu = psu.New(psu.Config{ComPort: s.ComPort, Baud: s.Baud, Address: byte(s.Address)})
+	_ = a.psu.Start()
+}
+
+// GetPSUStatus returns the supply's last polled state for the UI.
+func (a *App) GetPSUStatus() psu.Status {
+	if a.psu == nil {
+		return psu.Status{}
+	}
+	return a.psu.GetStatus()
+}
+
+// SetPSUOutput switches the supply's output. The only write OpsLog makes to it.
+func (a *App) SetPSUOutput(on bool) error {
+	if a.psu == nil {
+		return fmt.Errorf("the power supply is not enabled in Settings")
+	}
+	return a.psu.SetOutput(on)
+}