Implement DFS-based cycle detection in ValidateMolecule to catch
circular dependencies in molecule step graphs. The algorithm uses
three-color marking (unvisited/visiting/visited) to detect back
edges that indicate cycles.
When a cycle is detected, the error message shows the cycle path
(e.g., "a -> b -> c -> a") for easy debugging.
Add 4 new tests:
- SimpleCycle: A -> B -> A
- LongerCycle: A -> B -> C -> A
- DiamondNoCycle: ensures valid diamond patterns pass
- CycleInSubgraph: cycle not involving root node
Closes gt-ai1z.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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