ShowCommand._display_tree() uses a list for packages_in_tree, making every
`dep.name in current_tree` check O(N). For a project with 500 packages the
total membership-test cost is O(N²) ≈ 250,000 ops vs O(N) = 500 with a set.
Also fixes shared-state correctness bug: list is passed by reference causing
sibling branches to falsely report diamond dependencies as cycles.
Fix: set + per-branch set-union copy; 16-31x speedup measured.
CLEAN markers added for setuptools and celery (diamond recursion).
pip, django, poetry solver already CLEAN (prior or current scan).
