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/wsjt.go

@@ -523,10 +523,24 @@ var decodeModeChar = map[string]string{
 	"#": "JT65",
 	"@": "JT9",
 	"&": "MSK144",
-	":": "Q65",
 	"`": "FST4",
 }
 
+// ambiguousModeChar is a marker the forks do not agree on.
+//
+// ":" is Q65 in WSJT-X and FT4 in JTDX, so the character alone cannot answer:
+// a JTDX operator decoding FT4 was told they were on Q65, and every verdict
+// downstream — new mode, new slot, the mode filter — was computed against a
+// mode nobody was using.
+//
+// The sender's own Status settles it. It comes from the same program, names the
+// mode in full, and is re-sent whenever it changes, so it knows what that
+// program is decoding in a way one character never can. The fallback is
+// WSJT-X's reading, which is the older and commoner one.
+var ambiguousModeChar = map[string]string{
+	":": "Q65",
+}
+
 // DecodeModeName resolves a Decode's mode field to a real mode name. statusMode
 // is the mode from the same program's last Status, used when the field is a
 // marker we do not know, or empty.
@@ -535,6 +549,13 @@ func DecodeModeName(raw, statusMode string) string {
 	if m, ok := decodeModeChar[raw]; ok {
 		return m
 	}
+	if fallback, ok := ambiguousModeChar[raw]; ok {
+		// Believe the program over the character it happened to print.
+		if st := strings.ToUpper(strings.TrimSpace(statusMode)); st != "" {
+			return st
+		}
+		return fallback
+	}
 	// A mode name is at least two alphanumeric characters ("FT8", "JS8", "Q65").
 	// Anything shorter, or carrying punctuation, is a marker rather than a name.
 	if len(raw) >= 2 {