Establish automated test foundation

docs/design/test-foundation.md

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+# Test Foundation
+
+## Problem
+
+The crate parser handles unusual binary text and known extension artifacts. Without
+automated tests, a small refactor could silently change library counts or reports.
+
+## Architecture
+
+Tests cover the read-only pipeline from crate and filesystem inputs through the
+core library model to CSV, text, and CLI output. All test data is synthetic and is
+created in temporary directories.
+
+## Edge Cases
+
+- Known MP3, M4A, WAV, and AIF decode artifacts.
+- Crate records without a recognized stop marker.
+- Supported extensions with mixed case and unsupported files.
+- References that exist by filename and references that remain missing.
+
+## Verification
+
+Run `pytest`. No test reads a real Serato library or writes outside pytest's
+temporary directory.