fix(kpa): stop switching the KPA500 off, and answer its operator instantly

Three faults, one report. The slow poll asks ^TP — the KPA1500's ATU, which
a KPA500 (no ATU) never answers — and ask() dropped the whole connection on
any read timeout: a two-second stall and a reconnect every slow cycle, which
is why buttons lagged and the status read wrong. Worse, every serial reopen
toggled DTR/RTS — and those lines are the KPA500's POWER SWITCH (that is how
the Elecraft utility turns it on), so the amplifier obediently switched off
twenty seconds after its operator pressed nothing but Standby.

Silence is no longer a dead link (write errors still are), ^TP is never asked
again after one silence, the control lines are asserted once and held, and
the baud field becomes a list of the rates these amplifiers actually speak.

changelog.json

@@ -1,4 +1,14 @@
 [
+  {
+    "version": "0.27.3",
+    "date": "",
+    "en": [
+      "KPA500: the amplifier no longer switches itself off and commands respond instantly. A command this model does not know (the KPA1500’s ATU poll) was tearing the link down every cycle, and each reconnect toggled the serial control lines — which are the KPA500’s power switch. The lines are now held steady, silence is not treated as a dead link, and the baud is picked from a list."
+    ],
+    "fr": [
+      "KPA500 : l’ampli ne s’éteint plus tout seul et les commandes répondent instantanément. Une commande inconnue de ce modèle (le poll ATU du KPA1500) détruisait le lien à chaque cycle, et chaque reconnexion basculait les lignes de contrôle série — qui sont l’interrupteur du KPA500. Les lignes sont désormais tenues stables, le silence n’est plus traité comme un lien mort, et le baud se choisit dans une liste."
+    ]
+  },
   {
     "version": "0.27.2",
     "date": "",