test: pin which modes produce a recording

CW has moved in and out of this list twice — excluded for the missing sidetone,
restored because the other station is captured regardless. A test says which,
so the next reading of that comment does not undo the decision.

recordable_mode_test.go

@@ -0,0 +1,22 @@
+package main
+
+import "testing"
+
+// Which modes produce a recording worth keeping.
+//
+// CW has moved in and out of this list: it was excluded because SmartSDR does
+// not route the operator's own sidetone through DAX, then restored because the
+// other station is captured all the same. Pinned so it does not drift back.
+func TestRecordableMode(t *testing.T) {
+	for _, m := range []string{"SSB", "USB", "LSB", "AM", "FM", "DV", "CW", "cw", " CW "} {
+		if !recordableMode(m) {
+			t.Errorf("recordableMode(%q) = false — it should be recorded", m)
+		}
+	}
+	// Digital audio is a modem tone nobody replays.
+	for _, m := range []string{"FT8", "FT4", "RTTY", "PSK31", "JT65", "", "DIGU"} {
+		if recordableMode(m) {
+			t.Errorf("recordableMode(%q) = true — digital modes carry no useful audio", m)
+		}
+	}
+}