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.

internal/cluster/idle_test.go

@@ -0,0 +1,89 @@
+package cluster
+
+import (
+	"net"
+	"strconv"
+	"testing"
+	"time"
+)
+
+// A quiet cluster must not be treated as a broken one.
+//
+// Reported by an operator whose node sends little: nothing for two minutes and
+// OpsLog reconnected — losing the login, the filters, and any spot that landed
+// during the gap. The read deadline was being read as a verdict on the link
+// rather than as a way to stay responsive to a stop request.
+//
+// The test holds the connection open, silent for longer than one deadline, then
+// sends a spot. A session that survives receives it; one that reconnects does
+// not, because this listener never accepts twice.
+func TestSilenceDoesNotEndTheSession(t *testing.T) {
+	// Shorten the tick so the test exercises the real code path in seconds.
+	defer func(orig time.Duration) { idleTick = orig }(idleTick)
+	idleTick = time.Second
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+
+	accepted := make(chan net.Conn, 2)
+	go func() {
+		for {
+			c, err := ln.Accept()
+			if err != nil {
+				return
+			}
+			accepted <- c
+		}
+	}()
+
+	host, portStr, _ := net.SplitHostPort(ln.Addr().String())
+	port, err := strconv.Atoi(portStr)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	spots := make(chan Spot, 4)
+	s := &session{
+		cfg:      ServerConfig{Name: "quiet", Host: host, Port: port},
+		onSpot:   func(sp Spot) { spots <- sp },
+		onLine:   func(Line) {},
+		onStatus: func() {},
+		stopCh:   make(chan struct{}),
+	}
+
+	done := make(chan error, 1)
+	go func() { _, err := s.runOnce(); done <- err }()
+
+	conn := <-accepted
+	defer conn.Close()
+
+	// Silent for longer than one deadline: the loop must ride through it.
+	time.Sleep(3 * idleTick)
+
+	select {
+	case c := <-accepted:
+		c.Close()
+		t.Fatal("the session reconnected during the silence")
+	case err := <-done:
+		t.Fatalf("the session ended during the silence: %v", err)
+	default:
+	}
+
+	// Now a spot: it proves the ORIGINAL connection is still the live one.
+	if _, err := conn.Write([]byte("DX de F4BPO:     14074.0  OY1CT        FT8            1234Z\r\n")); err != nil {
+		t.Fatal(err)
+	}
+	select {
+	case sp := <-spots:
+		if sp.DXCall != "OY1CT" {
+			t.Errorf("spot from the wrong station: %+v", sp)
+		}
+	case <-time.After(5 * time.Second):
+		t.Fatal("the spot sent after the silence never arrived")
+	}
+
+	close(s.stopCh)
+	<-done
+}