Split index tests into smaller suites

[BelfordZ]

Summary

• move helper to src/test-utils.ts
• split large src/index.test.ts into:
  • basic.test.ts
  • cycle-detection.test.ts
  • traversal-options.test.ts
  • mutability.test.ts
• remove original monolithic test file

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_68550cad16a4832faa7c73d9f3da04ca

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1f704fe) to head (dc30d67).
Report is 13 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #819   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         2    +1     
  Lines          104       112    +8     
  Branches        38        43    +5     
=========================================
+ Hits           104       112    +8     

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