fix: a quiet cluster was treated as a dead one and reconnected

Reported by an operator whose node sends little: nothing for two minutes, then a
reconnect — losing the login, the filters, and any spot that arrived during the
gap.

The read had a 120 s deadline and ANY error ended the session, a timeout
included. That reads a silence as a verdict on the link. It is not: a cluster on
a dead band legitimately says nothing for minutes.

Only a real error ends a session now. A genuinely dead peer is still caught, and
by the mechanism meant for it — TCP keepalive probes an idle connection and its
failure surfaces as an error on Read, not as a timeout. The dialer sets it
explicitly rather than inheriting a default, since it is now what the design
depends on.

One trap the advice this came from did not mention: ReadString returns the bytes
it HAD read along with the timeout. Discarding them would lose the first half of
any spot line straddling a deadline, so the partial is carried over.

Tested: a listener that goes silent past the deadline and then sends a spot —
the spot arrives on the ORIGINAL connection, and the listener never accepts a
second one. The tick is a var so that test runs in four seconds instead of
sixty; a slow test is a test that gets skipped.

changelog.json

@@ -11,7 +11,8 @@
       "The padlocks on frequency, band, mode and times no longer flicker under the pointer and refuse the click.",
       "The callsign field is wider, taking the room from the RST pair.",
       "MP3 recordings no longer carry a hiss of their own: the 16→32 kHz conversion mirrored the whole spectrum only 7 dB down. It is now 57 dB down.",
-      "Replaying a QSO recording on the air has its own level, separate from the voice keyer: a recording comes from a receiver line output and needs far less gain than a message spoken into a microphone."
+      "Replaying a QSO recording on the air has its own level, separate from the voice keyer: a recording comes from a receiver line output and needs far less gain than a message spoken into a microphone.",
+      "A quiet DX cluster no longer looks like a broken one: silence stopped counting as a disconnection, so a node with few spots keeps its session, its login and its filters."
     ],
     "fr": [
       "Les filtres du cluster gagnent NEW POTA et NEW COUNTY, à côté des pastilles de statut existantes.",
@@ -22,7 +23,8 @@
       "Les cadenas de fréquence, bande, mode et heures ne scintillent plus sous le curseur et acceptent le clic.",
       "Le champ indicatif est plus large, la place venant de la paire RST.",
       "Les enregistrements MP3 ne portent plus un souffle qui leur est propre : la conversion 16→32 kHz recopiait tout le spectre à seulement 7 dB en dessous. Il est désormais à 57 dB.",
-      "La relecture d'un enregistrement de QSO a son propre niveau, distinct du manipulateur vocal : une prise vient de la sortie ligne d'un récepteur et demande bien moins de gain qu'un message dit au micro."
+      "La relecture d'un enregistrement de QSO a son propre niveau, distinct du manipulateur vocal : une prise vient de la sortie ligne d'un récepteur et demande bien moins de gain qu'un message dit au micro.",
+      "Un cluster calme ne passe plus pour un cluster mort : le silence ne compte plus comme une déconnexion, donc un nœud avec peu de spots garde sa session, son login et ses filtres."
     ]
   },
   {