refactor: move source files to src/ directory and rename project to Captioneer

Restructure Go source code into a dedicated `src/` directory, rename the project from "audio-listen" to "Captioneer", and update README accordingly. Add `.gitignore` entry for the built binary `captioneer`.

src/capture.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+	"sync"
+)
+
+func validatePrograms() error {
+	for _, program := range []string{"pactl", "parec"} {
+		if _, err := exec.LookPath(program); err != nil {
+			return fmt.Errorf("%s was not found in PATH: %w", program, err)
+		}
+	}
+	return nil
+}
+
+func defaultMonitorSource(ctx context.Context) (string, error) {
+	output, err := exec.CommandContext(ctx, "pactl", "get-default-sink").Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(output)) + ".monitor", nil
+}
+
+func capturePackets(ctx context.Context, monitorSource string) (<-chan []float32, func() error, error) {
+	cmd := exec.CommandContext(ctx, "parec",
+		"--device="+monitorSource,
+		"--format=s16le",
+		fmt.Sprintf("--rate=%d", sampleRate),
+		fmt.Sprintf("--channels=%d", channels),
+		"--raw",
+		"--latency-msec=50",
+	)
+	cmd.Stderr = os.Stderr
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, nil, err
+	}
+	if err := cmd.Start(); err != nil {
+		return nil, nil, err
+	}
+
+	packets := make(chan []float32, 50) // Five seconds at 100 ms per packet.
+	var waitOnce sync.Once
+	var waitErr error
+	wait := func() error {
+		waitOnce.Do(func() { waitErr = cmd.Wait() })
+		return waitErr
+	}
+
+	go func() {
+		defer close(packets)
+		packetBytes := make([]byte, samplesPerPacket()*bytesPerSample)
+		for {
+			n, readErr := io.ReadFull(stdout, packetBytes)
+			if n > 0 {
+				enqueueLatest(packets, pcm16LEToFloat32(packetBytes[:n]))
+			}
+			if readErr != nil {
+				if ctx.Err() == nil && !errors.Is(readErr, io.EOF) && !errors.Is(readErr, io.ErrUnexpectedEOF) {
+					fmt.Fprintf(os.Stderr, "audio capture read error: %v\n", readErr)
+				}
+				return
+			}
+		}
+	}()
+	return packets, wait, nil
+}
+
+func enqueueLatest(packets chan []float32, packet []float32) {
+	select {
+	case packets <- packet:
+		return
+	default:
+	}
+
+	// Keep the newest audio so captions recover quickly after overload.
+	select {
+	case <-packets:
+	default:
+	}
+	select {
+	case packets <- packet:
+	default:
+	}
+	fmt.Fprintln(os.Stderr, "warning: caption processing overloaded; dropped 100 ms of oldest audio")
+}