Make the documented zero-dependency test path runnable and safe

[protostatis]

Summary

The package declares no dependencies and the README says zero dependencies, but test_pyreplab.sh unconditionally imports pandas and numpy. In a clean environment, the test suite aborts at the pandas display test with ModuleNotFoundError: No module named pandas.

test_flags.sh cleanup also uses a broad pgrep plus kill -9 fallback that can terminate unrelated pyreplab daemons in a developer environment.

Expected

• Core tests pass with only the declared stdlib requirements, or optional pandas/numpy assertions skip clearly when unavailable.
• Test cleanup only terminates PIDs and session directories created by that test.

Reproduction

Running bash test_pyreplab.sh in the reviewed clean environment stopped at test 8 because pandas was unavailable.

Suggested direction

Guard optional-library cases, document a test extra if desired, and remove unscoped process-kill cleanup.