Establish automated test foundation

tests/test_scanner.py

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+from serato_doctor.scanner import scan_audio
+
+
+def test_scan_audio_finds_supported_files(tmp_path):
+    music = tmp_path / "music"
+    nested = music / "House"
+    nested.mkdir(parents=True)
+    (nested / "First.MP3").write_bytes(b"synthetic audio")
+    (nested / "Second.flac").write_bytes(b"fixture")
+    (nested / "notes.txt").write_text("not audio", encoding="utf-8")
+
+    tracks = scan_audio(music)
+
+    assert {track.filename for track in tracks} == {"First.MP3", "Second.flac"}
+    first = next(track for track in tracks if track.filename == "First.MP3")
+    assert first.suffix == ".mp3"
+    assert first.size == len(b"synthetic audio")