Enhance slab tensile criterion logic and update tests

[pillowbeast]

• Bugfix tau sign
• Updated the slab tensile criterion calculation to conditionally exclude low-density layers based on downward failure growth.
• Improved docstrings for clarity on low-density threshold behavior.
• Added tests to verify the correct application of the updated tensile criterion logic.
• Adjusted comparison cases in tests for consistency with new logic.

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