Add BFS cycle tests

[BelfordZ]

Summary

• cover cycle handling in bfs mode
• verify bfs works with skip-first and cyclical refs

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_68550d0a8428832f98ea7b83c2085c14

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (985a969) to head (8369879).
Report is 3 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #817   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          115       115           
  Branches        42        42           
=========================================
  Hits           115       115           

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