Add opt-in diagnostic logging

tests/test_cli.py

@@ -37,7 +37,9 @@ def test_cli_writes_reports_and_prints_counts(tmp_path, monkeypatch, capsys):
 
     main()
 
-    output = capsys.readouterr().out
+    captured = capsys.readouterr()
+    output = captured.out
+    assert captured.err == ""
     assert "Crate references: 2" in output
     assert "Disk tracks: 1" in output
     assert "Missing by filename: 1" in output