Closes #701

Summary

• Extracted a reusable edge helper so dependencies and conditional branches share identical adjacency logic, lowering BuildGraph cognitive complexity and future maintenance cost.
• Added focused XML docstrings around the graph helpers to document intent and keep the validator aligned with the project's documentation-centric conventions.
• Replaced the LINQ-based conditional branch enumeration with direct iteration for clarity while maintaining the existing workflow semantics.

Testing

• dotnet test FlowSynx.sln (fails: installed SDK only supports up to .NET 8 while the solution targets .NET 9)