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/audio/audio.go

@@ -0,0 +1,52 @@
+// Package audio contains the application's PCM and audio-level utilities.
+package audio
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+	"time"
+)
+
+const (
+	SampleRate     = 16000
+	Channels       = 1
+	BytesPerSample = 2
+	PacketDuration = 100 * time.Millisecond
+)
+
+func SamplesPerPacket() int {
+	return DurationSamples(PacketDuration)
+}
+
+func PCM16LEToFloat32(pcm []byte) []float32 {
+	samples := make([]float32, len(pcm)/BytesPerSample)
+	for i := range samples {
+		samples[i] = float32(int16(binary.LittleEndian.Uint16(pcm[i*2:]))) / 32768
+	}
+	return samples
+}
+
+func DurationSamples(duration time.Duration) int {
+	return int(duration * SampleRate / time.Second)
+}
+
+func RMSDBFS(samples []float32) float64 {
+	if len(samples) == 0 {
+		return math.Inf(-1)
+	}
+
+	var sum float64
+	for _, sample := range samples {
+		sum += float64(sample * sample)
+	}
+	rms := math.Sqrt(sum / float64(len(samples)))
+	if rms == 0 {
+		return math.Inf(-1)
+	}
+	return 20 * math.Log10(rms)
+}
+
+func FormatTime(samples int) string {
+	return fmt.Sprintf("%.2fs", float64(samples)/SampleRate)
+}

src/audio/audio_test.go

@@ -0,0 +1,40 @@
+package audio
+
+import (
+	"encoding/binary"
+	"math"
+	"testing"
+	"time"
+)
+
+func TestPCM16LEToFloat32(t *testing.T) {
+	pcm := make([]byte, 6)
+	binary.LittleEndian.PutUint16(pcm[0:], uint16(0))
+	binary.LittleEndian.PutUint16(pcm[2:], uint16(32767))
+	binary.LittleEndian.PutUint16(pcm[4:], uint16(32768))
+	got := PCM16LEToFloat32(pcm)
+	want := []float32{0, 32767.0 / 32768.0, -1}
+	for i := range want {
+		if math.Abs(float64(got[i]-want[i])) > 0.00001 {
+			t.Fatalf("sample %d = %f, want %f", i, got[i], want[i])
+		}
+	}
+}
+
+func TestDurationSamples(t *testing.T) {
+	if got := DurationSamples(time.Second); got != SampleRate {
+		t.Fatalf("one second = %d samples, want %d", got, SampleRate)
+	}
+	if got := DurationSamples(1500 * time.Millisecond); got != 24000 {
+		t.Fatalf("1.5 seconds = %d samples, want 24000", got)
+	}
+}
+
+func TestRMSDBFS(t *testing.T) {
+	if got := RMSDBFS([]float32{1, -1}); math.Abs(got) > 0.00001 {
+		t.Fatalf("full-scale RMS = %f dBFS, want 0", got)
+	}
+	if got := RMSDBFS([]float32{0, 0}); !math.IsInf(got, -1) {
+		t.Fatalf("silence = %f dBFS, want -Inf", got)
+	}
+}