Fix self-demo BATS fixture path portability

[codeforester]

Summary

• Replace the hard-coded personal base-bash-libs path in the self-demo BATS pause test with the existing portable fixture resolver.
• Add a contract check that prevents that personal path from returning to the BATS test.

Root Cause

The self-demo pause test used /Users/rameshhp/work/base-bash-libs/lib/bash as a fallback BASE_BASH_LIBS_DIR. That works on the primary macOS checkout but fails on fresh Ubuntu source checkouts, so the test exits before exercising the terminal-fd pause behavior.

Validation

• PYTHONPATH=/Users/rameshhp/work/base/lib/python:/Users/rameshhp/work/base/cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest tests/test_contract_hardening.py -q
• bats cli/bash/commands/basectl/tests/demo.bats
• git diff --check
• Prior full local gate after this patch: env -u BASE_HOME ./bin/base-test -> Python 731 passed, 1 skipped; BATS 569 ok

Fixes #1322