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/models/sherpa.go

@@ -130,8 +130,9 @@ func validatePaths(modelsDir string, mode config.Mode) (modelPaths, error) {
 func newVAD(model string, threads int) *sherpa.VoiceActivityDetector {
 	return sherpa.NewVoiceActivityDetector(&sherpa.VadModelConfig{
 		SileroVad: sherpa.SileroVadModelConfig{
-			Model:              model,
-			Threshold:          0.5,
+			Model:     model,
+			Threshold: 0.5,
+			// A 500 ms pause ends the current caption line and starts the next one.
 			MinSilenceDuration: 0.5,
 			MinSpeechDuration:  0.25,
 			WindowSize:         512,