refactor: extract audio helpers to domain/audio_helpers.py

- Extract build_ffmpeg_command, to_float32, apply_m4b_chapters_with_mutagen
- _apply_m4b_chapters_with_mutagen becomes thin wrapper with error handling
- Add tests/test_audio_helpers.py (12 tests)
- conversion_runner.py: 1410 → 1320 lines
- All tests pass

tests/test_audio_helpers.py

@@ -0,0 +1,127 @@
+"""Tests for audio helper utilities.
+
+Tests import from domain/audio_helpers.py (new module).
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import numpy as np
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# build_ffmpeg_command
+# ---------------------------------------------------------------------------
+
+
+class TestBuildFfmpegCommand:
+    """build_ffmpeg_command builds ffmpeg argument list."""
+
+    def test_base_structure(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out/audio.wav"), "wav")
+        assert cmd[0] == "ffmpeg"
+        assert "-y" in cmd
+        assert "pipe:0" in cmd
+        assert str(Path("/out/audio.wav")) in cmd
+
+    def test_mp3_codec(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out.mp3"), "mp3")
+        assert "libmp3lame" in cmd
+
+    def test_opus_codec(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out.opus"), "opus")
+        assert "libopus" in cmd
+
+    def test_m4b_codec(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out.m4b"), "m4b")
+        assert "aac" in cmd
+        assert "+faststart+use_metadata_tags" in cmd
+
+    def test_wav_copy_codec(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out.wav"), "wav")
+        assert "copy" in cmd
+
+    def test_with_metadata(self):
+        from abogen.domain.audio_helpers import build_ffmpeg_command
+
+        cmd = build_ffmpeg_command(Path("/out.mp3"), "mp3", metadata={"album": "Test"})
+        assert str(Path("/out.mp3")) in cmd
+
+
+# ---------------------------------------------------------------------------
+# to_float32
+# ---------------------------------------------------------------------------
+
+
+class TestToFloat32:
+    """to_float32 converts audio to float32 numpy array."""
+
+    def test_none_returns_empty(self):
+        from abogen.domain.audio_helpers import to_float32
+
+        result = to_float32(None)
+        assert isinstance(result, np.ndarray)
+        assert result.dtype == np.float32
+        assert len(result) == 0
+
+    def test_numpy_array(self):
+        from abogen.domain.audio_helpers import to_float32
+
+        arr = np.array([1.0, 2.0, 3.0], dtype="float64")
+        result = to_float32(arr)
+        assert result.dtype == np.float32
+        assert len(result) == 3
+
+    def test_mock_tensor(self):
+        from abogen.domain.audio_helpers import to_float32
+
+        tensor = MagicMock()
+        tensor.detach.return_value = tensor
+        tensor.cpu.return_value = tensor
+        tensor.numpy.return_value = np.array([1.0, 2.0])
+        result = to_float32(tensor)
+        assert result.dtype == np.float32
+        assert len(result) == 2
+
+    def test_list_input(self):
+        from abogen.domain.audio_helpers import to_float32
+
+        result = to_float32([1.0, 2.0])
+        assert result.dtype == np.float32
+        assert len(result) == 2
+
+
+# ---------------------------------------------------------------------------
+# apply_m4b_chapters_with_mutagen
+# ---------------------------------------------------------------------------
+
+
+class TestApplyM4bChaptersWithMutagen:
+    """apply_m4b_chapters_with_mutagen writes chapter atoms to MP4."""
+
+    def test_empty_chapters_returns_false(self):
+        from abogen.domain.audio_helpers import apply_m4b_chapters_with_mutagen
+
+        assert apply_m4b_chapters_with_mutagen(Path("/fake.m4b"), []) is False
+
+    def test_missing_mutagen_raises(self):
+        from abogen.domain.audio_helpers import apply_m4b_chapters_with_mutagen
+
+        with patch.dict("sys.modules", {"mutagen": None, "mutagen.mp4": None}):
+            with pytest.raises((ImportError, KeyError)):
+                apply_m4b_chapters_with_mutagen(
+                    Path("/fake.m4b"), [{"start": 0, "title": "Ch1"}]
+                )