fix(amps): combined amplifiers are commanded together, power level included

Two faults in the combiner coupling, both reported from the operating position.

THE POWER LEVEL WAS NEVER COUPLED. ON, OFF and OPERATE fanned out to the group;
L/M/H did not — it was simply the command nobody had linked. Two combined
amplifiers left at different power levels feed the combiner unevenly, which is
the thing the coupling exists to prevent.

THE COMMANDS DID NOT LEAVE TOGETHER. The second amplifier was commanded only
once the first had answered, and an SPE answers over its own link in its own
time. The combiner heard power appear on one input before the other and beeped
about it, on every OFF and every ON.

Each target now gets a goroutine, all parked on one channel until every one is
ready; closing it releases them together. That is the difference between "start
one, then start the other" and "both leave at once" — they have separate clients
and separate connections, so nothing downstream re-serialises them.

It matters most on the power level, which is not one command at all: an SPE has
no "set level", so the driver taps the POWER key and waits for the amp to report
the new one before tapping again — up to three taps, up to two seconds each. One
after the other, the pair would sit at different levels for six seconds.

A single amplifier still runs inline: no goroutine, no barrier, nothing new to
go wrong for the operators who have one amp. The one that was clicked stays
first, because its failure is the one worth reporting.

changelog.json

@@ -9,7 +9,8 @@
       "TCI radios: when the rig forbids transmitting, PTT says so instead of doing nothing silently.",
       "WAJA carried Japan’s civil prefecture numbers instead of the JARL’s: 35 of the 47 references are renumbered.",
       "Award references can be renumbered in the editor — the number was the one field it would not let you correct.",
-      "The compass fills the moment Station Control opens, instead of waiting out the rest of a polling interval."
+      "The compass fills the moment Station Control opens, instead of waiting out the rest of a polling interval.",
+      "Combined amplifiers: the power level (L/M/H) is coupled too, and both amps are commanded at once so the combiner stops beeping."
     ],
     "fr": [
       "Une entité qui est un seul groupe d’îles remplit désormais la référence IOTA toute seule, sans abonnement callbook.",
@@ -18,7 +19,8 @@
       "Radios TCI : quand la radio interdit l’émission, le PTT le dit au lieu de ne rien faire en silence.",
       "WAJA portait les numéros civils des préfectures japonaises et non ceux de la JARL : 35 des 47 références sont renumérotées.",
       "Les références d’un diplôme se renumérotent dans l’éditeur : le numéro était le seul champ qu’il refusait de corriger.",
-      "La boussole se remplit dès l’ouverture de Station Control, au lieu d’attendre la fin d’un intervalle d’interrogation."
+      "La boussole se remplit dès l’ouverture de Station Control, au lieu d’attendre la fin d’un intervalle d’interrogation.",
+      "Amplis combinés : le niveau de puissance (L/M/H) est couplé lui aussi, et les deux amplis sont commandés en même temps — fini le bip du combineur."
     ]
   },
   {