fix(udp): ':' is Q65 to WSJT-X and FT4 to JTDX

Reported from a real shack: JTDX decoding FT4 came through as Q65. A
Decode carries a one-character marker rather than a mode name, and the
forks do not agree on that one — so the character alone cannot answer,
and its wrong answer reached everything behind it: new mode, new slot,
the mode filter, all computed against a mode nobody was using.

The sending program's own Status settles it. It comes from that program,
names the mode in full, and is re-sent whenever it changes, so it knows
what is being decoded in a way one character never can. Only the
ambiguous marker consults it: the ones both forks agree on keep
answering from the table, Status or no Status, and with no Status at all
':' still reads as Q65 — WSJT-X's meaning, the older and commoner.

internal/integrations/udp/decodemode_test.go

@@ -113,3 +113,27 @@ func TestDecodeKeepsTheRawModeMarkerForReplies(t *testing.T) {
 			ev.DecodeModeRaw, "~")
 	}
 }
+
+// Reported from a real shack: JTDX decoding FT4 showed up as Q65.
+//
+// The two forks disagree about ":" — Q65 in WSJT-X, FT4 in JTDX — so the
+// character alone cannot answer, and the wrong answer poisons every verdict
+// that follows: new mode, new slot, the mode filter. The program's own Status
+// names the mode in full and settles it.
+func TestAmbiguousModeCharDefersToTheProgram(t *testing.T) {
+	cases := []struct {
+		raw, status, want, why string
+	}{
+		{":", "FT4", "FT4", "JTDX decoding FT4"},
+		{":", "Q65", "Q65", "WSJT-X decoding Q65"},
+		{":", "", "Q65", "no Status yet — WSJT-X's reading, the older and commoner"},
+		// The unambiguous markers are unaffected, Status or no Status.
+		{"~", "FT4", "FT8", "a tilde is FT8 in both"},
+		{"+", "", "FT4", "a plus is FT4 in both"},
+	}
+	for _, c := range cases {
+		if got := DecodeModeName(c.raw, c.status); got != c.want {
+			t.Errorf("DecodeModeName(%q, %q) = %q, want %q — %s", c.raw, c.status, got, c.want, c.why)
+		}
+	}
+}