feat(cat): Elecraft K3/K4 DATA A — send MD6 + DT0 on data mode (from the stale branch)

Integrates the fix/jtdx-kenwood-tx work onto main: the Kenwood/Elecraft backend's
"data" data-mode path now sends MD6 then DT0 so a K3/K4 lands in DATA A — the
audio sub-mode FT8 uses. MD6 alone could leave the rig in an FSK/PSK sub-mode
where it keyed but the rear sound-card audio never modulated ("transmits but
nothing comes out"). The data-mode option is relabelled "DATA A — MD6+DT0
(Elecraft K3/K4)". A shared Kenwood/Elecraft backend is kept (the K3 emulates the
Kenwood dialect); the Elecraft specifics live behind the data-mode seam rather
than a 7000-line duplicate backend.

internal/cat/kenwood.go

@@ -395,7 +395,18 @@ func (k *Kenwood) SetMode(mode string) error {
 		case "keep":
 			return nil // leave whatever data mode the operator set on the rig
 		case "data":
-			return k.write("MD6;") // Elecraft K3/K4 DATA mode
+			// Elecraft K3/K4 DATA mode (MD6) PLUS the DATA-A submode (DT0). MD6 alone
+			// can leave the rig in an FSK/PSK data submode (from a prior RTTY/PSK
+			// session), where FT8 keys the transmitter but the rear sound-card audio
+			// never modulates — "transmits but nothing comes out". DT0 forces DATA A,
+			// the audio submode FT8 needs. This is the Elecraft path (the option is
+			// labelled K3/K4); a real K3/K4 answers DT with nothing. A plain Kenwood
+			// has no DT command and would "?;" it — but that's a misconfiguration
+			// (pick USB there), and IF is never latched so the link self-recovers.
+			if err := k.write("MD6;"); err != nil {
+				return err
+			}
+			return k.write("DT0;")
 		}
 	}
 	d := kenwoodModeDigit(mode, k.curFreq)