@spec struct_to_graph(Paradigm.Graph.t(), struct(), Paradigm.Graph.node_id()) ::
  Paradigm.Graph.t()

Converts an Elixir struct into graph nodes using the Graph protocol.

• Uses the struct's module name as the class_id
• Converts nested structs into references
• Inserts all nodes into the graph

@spec to_struct(Paradigm.Graph.t(), Paradigm.Graph.node_id()) :: struct() | nil

Converts a graph node into its canonical Elixir struct representation.

• The struct name is derived from the node's class
• All %Paradigm.Graph.Node.Ref{} values are expanded in place
• Cycle detection prevents infinite recursion