feat(architecture): extract code2graph package and integrate with code2llm models

[tom-sapletta-com]

Summary

• Extracted domain data models and topology queries into standalone package code2graph (v0.1.0) under packages/code2graph.
• code2graph provides:
  • Base models and dataclasses: AnalysisResult, FunctionInfo, ClassInfo, ModuleInfo, FlowNode, FlowEdge, Pattern, CodeSmell, Mutation, DataFlow, BaseModel.
  • In-memory graph traversal query engine: GraphQuery (BFS callers/callees, entry points, isolation).
• Integrated into code2llm/core/models.py with transparent fallback to embedded dataclasses if code2graph is unavailable.
• Fixed Rust call graph edge resolution to preserve unresolvable calls and retain caller declaration order for entry points.
• Added code2graph to pyproject.toml optional dependencies ([project.optional-dependencies] graph) and dev dependencies.
• All 355 pytest tests passing.