refactor: reorganize monolithic main package into focused packages

Move the executable entry point to `src/cmd/captioneer` and split audio, capture, config, caption, and model logic into separate packages under `src/`. Update README with new build/run commands and a package-overview section.

src/config/config.go

@@ -0,0 +1,51 @@
+// Package config defines Captioneer's command-line configuration.
+package config
+
+import (
+	"errors"
+	"flag"
+	"time"
+)
+
+const DefaultModelsDir = "models"
+
+type Mode string
+
+const (
+	ModeFixed Mode = "fixed"
+	ModeVAD   Mode = "vad"
+)
+
+type Settings struct {
+	Mode               Mode
+	ChunkDuration      time.Duration
+	ModelsDir          string
+	Threads            int
+	PreviewThresholdDB float64
+}
+
+func Parse() Settings {
+	var settings Settings
+	mode := ""
+	flag.StringVar(&mode, "mode", "", "caption mode: vad or fixed (required)")
+	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.Parse()
+	settings.Mode = Mode(mode)
+	return settings
+}
+
+func (s Settings) Validate() error {
+	if s.Mode != ModeFixed && s.Mode != ModeVAD {
+		return errors.New("--mode is required and must be either fixed or vad")
+	}
+	if s.ChunkDuration <= 0 {
+		return errors.New("--chunk-duration must be positive")
+	}
+	if s.Threads <= 0 {
+		return errors.New("--threads must be positive")
+	}
+	return nil
+}