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/models/sherpa.go

@@ -0,0 +1,149 @@
+// Package models owns Sherpa-Onnx model setup and inference resources.
+package models
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	sherpa "github.com/k2-fsa/sherpa-onnx-go-linux"
+	"tea.chunkbyte.com/kato/captioneer/src/audio"
+	"tea.chunkbyte.com/kato/captioneer/src/config"
+)
+
+type SpeechSegment struct {
+	Start   int
+	Samples []float32
+}
+
+type modelPaths struct {
+	encoder string
+	decoder string
+	joiner  string
+	tokens  string
+	vad     string
+}
+
+type Resources struct {
+	recognizer *sherpa.OfflineRecognizer
+	vad        *sherpa.VoiceActivityDetector
+}
+
+func New(settings config.Settings) (*Resources, error) {
+	paths, err := validatePaths(settings.ModelsDir, settings.Mode)
+	if err != nil {
+		return nil, err
+	}
+
+	recognizer := sherpa.NewOfflineRecognizer(&sherpa.OfflineRecognizerConfig{
+		FeatConfig: sherpa.FeatureConfig{SampleRate: audio.SampleRate, FeatureDim: 80},
+		ModelConfig: sherpa.OfflineModelConfig{
+			Transducer: sherpa.OfflineTransducerModelConfig{
+				Encoder: paths.encoder,
+				Decoder: paths.decoder,
+				Joiner:  paths.joiner,
+			},
+			Tokens:     paths.tokens,
+			NumThreads: settings.Threads,
+			Provider:   "cpu",
+			ModelType:  "nemo_transducer",
+		},
+		DecodingMethod: "greedy_search",
+	})
+	if recognizer == nil {
+		return nil, fmt.Errorf("create Parakeet recognizer: Sherpa returned nil")
+	}
+
+	resources := &Resources{recognizer: recognizer}
+	if settings.Mode == config.ModeVAD {
+		resources.vad = newVAD(paths.vad, settings.Threads)
+		if resources.vad == nil {
+			resources.Close()
+			return nil, fmt.Errorf("create Silero VAD: Sherpa returned nil")
+		}
+	}
+	return resources, nil
+}
+
+func (r *Resources) Close() {
+	if r.vad != nil {
+		sherpa.DeleteVoiceActivityDetector(r.vad)
+		r.vad = nil
+	}
+	if r.recognizer != nil {
+		sherpa.DeleteOfflineRecognizer(r.recognizer)
+		r.recognizer = nil
+	}
+}
+
+func (r *Resources) Decode(samples []float32) string {
+	if len(samples) == 0 {
+		return ""
+	}
+	stream := sherpa.NewOfflineStream(r.recognizer)
+	defer sherpa.DeleteOfflineStream(stream)
+	stream.AcceptWaveform(audio.SampleRate, samples)
+	r.recognizer.Decode(stream)
+	return strings.TrimSpace(stream.GetResult().Text)
+}
+
+func (r *Resources) AcceptVAD(samples []float32) {
+	r.vad.AcceptWaveform(samples)
+}
+
+func (r *Resources) FlushVAD() {
+	r.vad.Flush()
+}
+
+func (r *Resources) NextSpeechSegment() (SpeechSegment, bool) {
+	if r.vad.IsEmpty() {
+		return SpeechSegment{}, false
+	}
+	segment := r.vad.Front()
+	r.vad.Pop()
+	return SpeechSegment{Start: segment.Start, Samples: segment.Samples}, true
+}
+
+func validatePaths(modelsDir string, mode config.Mode) (modelPaths, error) {
+	parakeetDir := filepath.Join(modelsDir, "parakeet-tdt-v2")
+	paths := modelPaths{
+		encoder: filepath.Join(parakeetDir, "encoder.int8.onnx"),
+		decoder: filepath.Join(parakeetDir, "decoder.int8.onnx"),
+		joiner:  filepath.Join(parakeetDir, "joiner.int8.onnx"),
+		tokens:  filepath.Join(parakeetDir, "tokens.txt"),
+		vad:     filepath.Join(modelsDir, "silero_vad_v5.onnx"),
+	}
+	required := []string{paths.encoder, paths.decoder, paths.joiner, paths.tokens}
+	if mode == config.ModeVAD {
+		required = append(required, paths.vad)
+	}
+
+	var missing []string
+	for _, path := range required {
+		info, err := os.Stat(path)
+		if err != nil || info.IsDir() {
+			missing = append(missing, path)
+		}
+	}
+	if len(missing) > 0 {
+		return modelPaths{}, fmt.Errorf("missing required model files:\n  %s\nRun ./scripts/download-models.sh", strings.Join(missing, "\n  "))
+	}
+	return paths, nil
+}
+
+func newVAD(model string, threads int) *sherpa.VoiceActivityDetector {
+	return sherpa.NewVoiceActivityDetector(&sherpa.VadModelConfig{
+		SileroVad: sherpa.SileroVadModelConfig{
+			Model:              model,
+			Threshold:          0.5,
+			MinSilenceDuration: 0.5,
+			MinSpeechDuration:  0.25,
+			WindowSize:         512,
+			MaxSpeechDuration:  30,
+		},
+		SampleRate: audio.SampleRate,
+		NumThreads: threads,
+		Provider:   "cpu",
+	}, 60)
+}