fix: serialize testing.invoke cwd handling

[codeforester]

Fixes #11

Summary

• serialize testing.invoke(cwd=...) calls while the process cwd is temporarily changed
• preserve cwd restoration around successful and failing invocations
• add concurrent regression coverage
• document the process-global cwd contract and limitation

Root cause

The helper uses os.chdir() so relative file I/O sees the requested fixture directory. Because cwd is process-global, concurrent helper calls could race without serialization.

Checks

• python -m pytest tests/test_testing.py
• python -m pytest — 154 passed, 4 subtests passed
• ./tests/validate.sh