fix(decodes): a decode's mode is a marker, not a mode name

Every station on an already-worked band was flagged NEW MODE.

A WSJT-X Decode does not carry the mode's name. It carries the
one-character marker from the decode line - "~" for FT8, "+" for FT4 - and
that character was passed straight through as though it were a mode. The
status resolver then compared "~" against the modes worked for the entity,
matched nothing, and concluded the mode had never been worked. Same cause
put "~ -07" in the comment of every decode spot pushed to the FlexRadio
panadapter, which nobody had traced back.

Resolved through a marker table, with the mode from the sender's last
Status as the fallback - Status is the message that carries the real name.
So an unlisted or future marker degrades to correct rather than to
nonsense, and a sender that puts the name in the field directly is believed
as-is. With neither available the mode is left empty, which makes the
resolver answer "worked": the safe side, since a wrong mode invents a
new-mode flag exactly as the marker did.

internal/integrations/udp/decodemode_test.go

@@ -0,0 +1,59 @@
+package udp
+
+import "testing"
+
+// A Decode does not carry the mode's NAME. It carries the one-character marker
+// from the decode line — "~" for FT8, "+" for FT4 — and that character used to
+// be passed on as if it were a mode. Everything downstream compared it against
+// the modes in the log, matched nothing, and reported every station on an
+// already-worked band as a NEW MODE.
+func TestDecodeModeNameResolvesTheMarker(t *testing.T) {
+	for raw, want := range map[string]string{
+		"~": "FT8",
+		"+": "FT4",
+		"#": "JT65",
+		"@": "JT9",
+	} {
+		if got := DecodeModeName(raw, "FT8"); got != want {
+			t.Errorf("DecodeModeName(%q) = %q, want %q", raw, got, want)
+		}
+	}
+}
+
+// A sender that puts the real name in the field is believed as-is — several do,
+// and the marker table must not get in their way.
+func TestDecodeModeNameKeepsARealName(t *testing.T) {
+	for _, raw := range []string{"FT8", "ft4", "JS8", "Q65"} {
+		if got := DecodeModeName(raw, ""); got == "" || got != upper(raw) {
+			t.Errorf("DecodeModeName(%q) = %q, want the name itself", raw, got)
+		}
+	}
+}
+
+// The safety net: an unknown marker falls back to the mode from the sender's
+// last Status, which always carries the real name. This is what keeps a future
+// or unlisted marker degrading to correct rather than to nonsense.
+func TestDecodeModeNameFallsBackToStatus(t *testing.T) {
+	if got := DecodeModeName("%", "FT4"); got != "FT4" {
+		t.Errorf("unknown marker resolved to %q, want the Status mode FT4", got)
+	}
+	if got := DecodeModeName("", "FT8"); got != "FT8" {
+		t.Errorf("empty mode resolved to %q, want the Status mode FT8", got)
+	}
+	// Nothing known at all is empty rather than a guess: an empty mode makes the
+	// status resolver answer "worked", which is the safe side — a wrong mode
+	// would invent a new-mode flag exactly as the marker did.
+	if got := DecodeModeName("%", ""); got != "" {
+		t.Errorf("with no Status mode the result was %q, want empty", got)
+	}
+}
+
+func upper(s string) string {
+	out := []rune(s)
+	for i, r := range out {
+		if r >= 'a' && r <= 'z' {
+			out[i] = r - 32
+		}
+	}
+	return string(out)
+}