add test fixtures for edge cases

[cs01]

Summary

• Adds 6 test fixtures covering important edge cases that verify correctness of negative index handling, math operations with negatives, string comparisons, and empty array operations
• Tests: string-slice-negative, array-slice-negative, math-negative, string-comparisons, empty-array-ops, modulo-negative

Test plan

• All 6 fixtures pass with npm test
• npm run verify:quick passes

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