docs: update README with VAD, new threshold option, and silence handling

Update README to clarify VAD mode behavior: provisional caption only after Silero detects speech, Parakeet idle during silence. Document new --vad-threshold option to control Silero speech confidence. Add troubleshooting tip for music/noise false detections.

src/config/config.go

@@ -10,6 +10,7 @@ import (
 
 const (
 	DefaultModelsDir            = "models"
+	DefaultVADThreshold         = 0.5
 	DefaultOverlayFontFamily    = "Sans"
 	MinimumOverlayFinalLifetime = 3 * time.Second
 	DefaultModelTimeout         = 30 * time.Second
@@ -29,6 +30,7 @@ type Settings struct {
 	ModelsDir            string
 	Threads              int
 	PreviewThresholdDB   float64
+	VADThreshold         float64
 	ModelAutoRestart     bool
 	ModelTimeout         time.Duration
 	ModelShutdownTimeout time.Duration
@@ -41,7 +43,8 @@ func Parse() Settings {
 	flag.DurationVar(&settings.ChunkDuration, "chunk-duration", time.Second, "fixed chunk size or VAD preview refresh interval")
 	flag.StringVar(&settings.ModelsDir, "models-dir", DefaultModelsDir, "directory containing downloaded Sherpa models")
 	flag.IntVar(&settings.Threads, "threads", 2, "CPU threads used by recognition and VAD")
-	flag.Float64Var(&settings.PreviewThresholdDB, "preview-threshold-dbfs", -45, "RMS dBFS threshold used to begin VAD previews")
+	flag.Float64Var(&settings.PreviewThresholdDB, "preview-threshold-dbfs", -45, "RMS dBFS gate below which recognition stays idle")
+	flag.Float64Var(&settings.VADThreshold, "vad-threshold", DefaultVADThreshold, "speech detection sensitivity from 0 to 1; higher rejects more non-speech")
 	flag.BoolVar(&settings.ModelAutoRestart, "model-auto-restart", true, "restart the isolated model worker after a crash or timeout")
 	flag.DurationVar(&settings.ModelTimeout, "model-timeout", DefaultModelTimeout, "maximum model startup or processing time; zero disables hang detection")
 	flag.DurationVar(&settings.ModelShutdownTimeout, "model-shutdown-timeout", DefaultModelShutdownTimeout, "time allowed for model flush and cleanup before force-killing the worker")
@@ -94,7 +97,8 @@ func ParseDesktop() DesktopSettings {
 	flags.DurationVar(&desktop.ChunkDuration, "chunk-duration", time.Second, "fixed chunk size or VAD preview refresh interval")
 	flags.StringVar(&desktop.ModelsDir, "models-dir", DefaultModelsDir, "directory containing downloaded Sherpa models")
 	flags.IntVar(&desktop.Threads, "threads", 2, "CPU threads used by recognition and VAD")
-	flags.Float64Var(&desktop.PreviewThresholdDB, "preview-threshold-dbfs", -45, "RMS dBFS threshold used to begin VAD previews")
+	flags.Float64Var(&desktop.PreviewThresholdDB, "preview-threshold-dbfs", -45, "RMS dBFS gate below which recognition stays idle")
+	flags.Float64Var(&desktop.VADThreshold, "vad-threshold", DefaultVADThreshold, "speech detection sensitivity from 0 to 1; higher rejects more non-speech")
 	flags.BoolVar(&desktop.ModelAutoRestart, "model-auto-restart", true, "restart the isolated model worker after a crash or timeout")
 	flags.DurationVar(&desktop.ModelTimeout, "model-timeout", DefaultModelTimeout, "maximum model startup or processing time; zero disables hang detection")
 	flags.DurationVar(&desktop.ModelShutdownTimeout, "model-shutdown-timeout", DefaultModelShutdownTimeout, "time allowed for model flush and cleanup before force-killing the worker")
@@ -125,6 +129,9 @@ func (s Settings) Validate() error {
 	if s.Threads <= 0 {
 		return errors.New("--threads must be positive")
 	}
+	if s.VADThreshold < 0 || s.VADThreshold > 1 {
+		return errors.New("--vad-threshold must be between 0 and 1")
+	}
 	if s.ModelTimeout < 0 {
 		return errors.New("--model-timeout cannot be negative")
 	}