feat(rust): add native pipeline path detection and call graph metrics calculation

[tom-sapletta-com]

Summary

• Implemented native Rust pipeline candidate path detection (pipelines::find_pipeline_paths):
  • Traverses directed call graphs to find longest execution paths.
  • Explores weakly connected components to extract multi-stage data pipelines.
  • Replaces slow NetworkX DAG longest path iterations and set copies with fast in-memory Rust structures.
• Implemented native call graph fan-in, fan-out, and caller map computation (calls::calculate_call_metrics):
  • Calculates unique callers and unique callees in a single linear pass.
  • Updates called_by references for downstream betweenness centrality and entry point queries.
• Integrated into code2llm/analysis/pipeline_detector.py and code2llm/analysis/call_graph_engine.py via native_bridge.py.
• All 364 tests passing.