fix: serial CW keyer PTT — default on, live-apply, and diagnostic log

Reported: on a CQ macro the rig drops to RX between words instead of holding TX
for the whole macro. Root cause is PTT not being held (usePTT off, or the RTS
PTT line not wired/honoured). Improvements:

- "Key PTT line" now defaults ON when the Serial engine is selected — without it
  the rig runs on semi-break-in and drops between words.
- The serial keyer's line options (PTT, key line, invert, lead/tail, speed) are
  now atomic and live-updatable: SaveWinkeyerSettings -> ApplySerialConfig applies
  them from the next character, no reconnect needed.
- Each transmission logs its PTT state / key line / lead/tail, so a rig that
  still won't hold TX can be diagnosed from the app log.

The macro itself is sent as one string (not fragmented), so PTT is genuinely
held across word gaps when usePTT is on — pending on-air confirmation via the log.

app.go

@@ -13245,6 +13245,14 @@ func (a *App) SaveWinkeyerSettings(s WinkeyerSettings) error {
 			return err
 		}
 	}
+	// Apply line-control changes (PTT keying, key line, invert, lead/tail, speed)
+	// to a running serial keyer immediately — no reconnect needed. Harmless when
+	// the keyer isn't the serial type or isn't connected.
+	if a.winkeyer != nil && s.Engine == "serial" {
+		cfg := s.Config
+		cfg.Type = "serial"
+		a.winkeyer.ApplySerialConfig(cfg)
+	}
 	return nil
 }