feat: add six-line caption cap and enforce 3s minimum final timeout

- Reduce default overlay-final-timeout from 4s to 3s and validate
  that non-zero timeout is at least 3s to prevent captions from
  disappearing too quickly.
- Limit completed captions to six lines and keep each visible for
  at least 3s, improving readability during fast speech.
- Update README to document new VAD behavior and changed defaults.

src/config/config.go

@@ -7,7 +7,10 @@ import (
 	"time"
 )
 
-const DefaultModelsDir = "models"
+const (
+	DefaultModelsDir            = "models"
+	MinimumOverlayFinalLifetime = 3 * time.Second
+)
 
 type Mode string
 
@@ -88,7 +91,7 @@ func ParseDesktop() DesktopSettings {
 	flags.IntVar(&desktop.Overlay.BottomMargin, "overlay-bottom-margin", 100, "distance from the monitor bottom in logical pixels")
 	flags.IntVar(&desktop.Overlay.MaxWidth, "overlay-max-width", 1200, "fixed caption width in logical pixels before the monitor safety cap")
 	flags.StringVar(&desktop.Overlay.Monitor, "overlay-monitor", "auto", "monitor selection: auto, numeric index, or connector name")
-	flags.DurationVar(&desktop.Overlay.FinalTimeout, "overlay-final-timeout", 4*time.Second, "how long a final caption remains; zero waits for replacement")
+	flags.DurationVar(&desktop.Overlay.FinalTimeout, "overlay-final-timeout", MinimumOverlayFinalLifetime, "completed-caption lifetime; zero keeps lines until the six-line cap removes them")
 	flags.BoolVar(&desktop.Overlay.ClickThrough, "overlay-click-through", true, "pass pointer input through the caption window")
 	flag.Parse()
 	desktop.Mode = Mode(mode)
@@ -135,6 +138,9 @@ func (s DesktopSettings) Validate() error {
 	if s.Overlay.FinalTimeout < 0 {
 		return errors.New("--overlay-final-timeout cannot be negative")
 	}
+	if s.Overlay.FinalTimeout > 0 && s.Overlay.FinalTimeout < MinimumOverlayFinalLifetime {
+		return errors.New("--overlay-final-timeout must be zero or at least 3s")
+	}
 	if s.Overlay.Monitor == "" {
 		return errors.New("--overlay-monitor cannot be empty")
 	}