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.

changelog.json

@@ -7,14 +7,16 @@
       "A QSO logged from WSJT-X, MSHV or a net now appears in Recent QSOs at once, instead of waiting for a delayed auto-upload to send it.",
       "New installs: the default QSL and recording e-mails end with a credit line and a link to OpsLog. Part of the template, so delete it if unwanted.",
       "Relay automatic control and band-change messages now follow the Band selector too, so a station without CAT switches its antenna when you change band.",
-      "Fixed OpsLog re-tuning its own rig from its own radio broadcasts, which dropped the CAT link on every JTDX or WSJT-X “Fake It” transmission."
+      "Fixed OpsLog re-tuning its own rig from its own radio broadcasts, which dropped the CAT link on every JTDX or WSJT-X “Fake It” transmission.",
+      "New device: a bench power supply on Modbus RTU (BSIDE, Wanptek and kin) — its output switched from Station Control, with volts, amps and watts."
     ],
     "fr": [
       "Clic droit : mettre à jour le comté US des contacts sélectionnés depuis la base ULS, pour remplacer un comté renommé ou supprimé.",
       "Un QSO logué depuis WSJT-X, MSHV ou un net apparaît aussitôt dans les QSO récents, sans attendre l’envoi d’un upload automatique différé.",
       "Nouvelles installations : les mails QSL et enregistrement par défaut finissent par une ligne de crédit et un lien vers OpsLog. Dans le modèle, supprimable.",
       "Le contrôle automatique des relais et les messages de changement de bande suivent aussi le champ Band : une station sans CAT commute enfin son antenne.",
-      "Corrigé : OpsLog réaccordait sa propre radio depuis ses propres diffusions, ce qui coupait le lien CAT à chaque émission JTDX ou WSJT-X en « Fake It »."
+      "Corrigé : OpsLog réaccordait sa propre radio depuis ses propres diffusions, ce qui coupait le lien CAT à chaque émission JTDX ou WSJT-X en « Fake It ».",
+      "Nouvel appareil : alimentation de laboratoire en Modbus RTU (BSIDE, Wanptek et similaires) — sortie commutée depuis Contrôle station, avec V, A et W."
     ]
   },
   {