test(adif): pin that an import invents no QSL routing

Reported as OpsLog defaulting QSL_SENT_VIA and QSL_RCVD_VIA to "E" on import.
It does not: recordToQSO reads the field and normalises it, so an absent, empty,
blank or unrecognised value gives an empty one. An "E" seen after an import came
from the file — Log4OM writes QSL_SENT_VIA:1>E on every record whether a card
was ever sent, which TestQSLSentViaDoesNotBecomeManager already pins.

Nothing to change, so this pins the half nothing covered: the ABSENCE case. The
opposite is an easy reach — electronic confirmation is the common case — and
defaulting to it would quietly rewrite the operator's own record of how their
cards travelled, which is the same class of fault as issue #16.

Compiles and vets clean; not executed here — this machine's Application Control
policy began blocking freshly built test binaries partway through the session.

internal/adif/qslvia_import_test.go

@@ -0,0 +1,41 @@
+package adif
+
+import (
+	"strings"
+	"testing"
+)
+
+// An import must never INVENT a routing method. The two "via" fields carry what
+// the file carries, and nothing at all when it carries nothing.
+//
+// This is worth pinning because the opposite is easy to reach for: a card that
+// was confirmed electronically is the common case, and defaulting to E would
+// quietly rewrite the operator's own record of how their cards actually
+// travelled. Where an E does show up after an import, it came from the source
+// file — Log4OM writes QSL_SENT_VIA:1>E on every record whether or not a card
+// was ever sent, which is pinned by TestQSLSentViaDoesNotBecomeManager.
+func TestImportInventsNoQSLRouting(t *testing.T) {
+	for name, rec := range map[string]string{
+		"absent":     "<CALL:5>OE6CLD<QSO_DATE:8>20260606<TIME_ON:4>1200<BAND:3>20m<MODE:2>CW<EOR>\n",
+		"empty":      "<CALL:5>OE6CLD<QSO_DATE:8>20260606<TIME_ON:4>1200<BAND:3>20m<MODE:2>CW<QSL_SENT_VIA:0><QSL_RCVD_VIA:0><EOR>\n",
+		"whitespace": "<CALL:5>OE6CLD<QSO_DATE:8>20260606<TIME_ON:4>1200<BAND:3>20m<MODE:2>CW<QSL_SENT_VIA:1> <QSL_RCVD_VIA:1> <EOR>\n",
+		// Not in the enumeration: dropped rather than passed through, or the
+		// export would write a value no other logger can read.
+		"unknown": "<CALL:5>OE6CLD<QSO_DATE:8>20260606<TIME_ON:4>1200<BAND:3>20m<MODE:2>CW<QSL_SENT_VIA:3>ZZZ<EOR>\n",
+	} {
+		var got Record
+		if err := Parse(strings.NewReader("<EOH>\n"+rec), func(r Record) error { got = r; return nil }); err != nil {
+			t.Fatalf("%s: parse: %v", name, err)
+		}
+		q, ok := recordToQSO(got)
+		if !ok {
+			t.Fatalf("%s: recordToQSO returned !ok", name)
+		}
+		if q.QSLSentVia != "" {
+			t.Errorf("%s: QSL_SENT_VIA = %q, want empty — the import made up a routing method", name, q.QSLSentVia)
+		}
+		if q.QSLRcvdVia != "" {
+			t.Errorf("%s: QSL_RCVD_VIA = %q, want empty — the import made up a routing method", name, q.QSLRcvdVia)
+		}
+	}
+}