fix(tests): build packaging-test wheels from a private source copy

[NCCU-Schultz-Lab]

Summary

Fixes the flaky wheel-build tests in tests/test_packaging.py, first noted in #129.

Root cause: every test ran python -m build in the repository root. setuptools writes its intermediate build files into build/ and *.egg-info in the source tree, so when pytest-xdist runs several of these tests at once, one build deletes the directory another build is still using:

error: [Errno 2] No such file or directory: 'build/bdist.linux-x86_64/wheel'

Fix: _build_wheel now copies the checkout into the test's tmp_path and builds from that copy, so every test has its own build directories.

• The copy leaves out things that should never end up in a test wheel: .git, old build/dist/*.egg-info output, cache folders, *.sif container images, and the local .plans symlink.
• Symlinks are copied as links, not followed.
• tests/ stays in the copy, so the "tests are not shipped" check still has a tests/ package to exclude.

This is a test-only change.

Testing

• Before: pytest -n 5 tests/test_packaging.py failed in 4 of 4 runs, on various tests. I also reproduced the build/bdist error directly by running four python -m build processes at the same time.
• After: 0 of 4 runs failed with -n 5, and the file passes when run serially (-n0).
• pre-commit run is clean.

Authorship

• Claude (Opus 5.5): code edits, review, and conceptual discussion
• Jonathan Schultz: overall vision, planning, review, and orchestration

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