fix(adif): a logged QSO carries its receive side

The importer has always read an absent BAND_RX/FREQ_RX as 'not split, so
RX = TX'. The logging paths never did, so what a contact carried
depended on which door it came in through — and it shows outside
OpsLog: the record forwarded over UDP is written from the QSO as logged,
so a receiver that reads BAND_RX (Log4OM does) found nothing there.

Filled at AddQSO, the one funnel every path goes through, so the
database, the export and the forwarded copy agree. A genuine split
contact keeps what it was given — table-tested both ways.

rxdefaults_test.go

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"hamlog/internal/adif"
+	"hamlog/internal/qso"
+)
+
+// A contact that was not split still has a receive side, and the record
+// forwarded to another logger has to carry it: Log4OM reads BAND_RX.
+func TestFillRXDefaults(t *testing.T) {
+	hz := int64(14074000)
+	q := qso.QSO{Callsign: "F4BPO", Band: "20m", FreqHz: &hz}
+	fillRXDefaults(&q)
+	if q.BandRX != "20m" {
+		t.Errorf("BandRX = %q, want 20m", q.BandRX)
+	}
+	if q.FreqRXHz == nil || *q.FreqRXHz != hz {
+		t.Errorf("FreqRXHz = %v, want %d", q.FreqRXHz, hz)
+	}
+	rec := adif.SingleRecordADIF(q)
+	if !strings.Contains(strings.ToUpper(rec), "<BAND_RX:3>20M") {
+		t.Errorf("BAND_RX missing from the forwarded record:\n%s", rec)
+	}
+}
+
+// A genuine split contact keeps what it was given.
+func TestFillRXDefaultsKeepsSplit(t *testing.T) {
+	tx, rx := int64(14195000), int64(14205000)
+	q := qso.QSO{Callsign: "F4BPO", Band: "20m", BandRX: "17m", FreqHz: &tx, FreqRXHz: &rx}
+	fillRXDefaults(&q)
+	if q.BandRX != "17m" || q.FreqRXHz == nil || *q.FreqRXHz != rx {
+		t.Errorf("split QSO was overwritten: band_rx=%q freq_rx=%v", q.BandRX, q.FreqRXHz)
+	}
+}