perf: avoid O(n^2) queue ops in DataFlowAnalyzer BFS

[omsherikar]

Summary

DataFlowAnalyzer._collect_nodes uses a Python list queue with pop(0), which is O(n) per pop. On large CFGs this turns node collection into avoidable O(n^2) behavior.

Suggested direction

• Replace list queue with collections.deque and popleft().
• Keep traversal behavior identical (reachable nodes only, deterministic ordering after final sort).

Acceptance

• Same node set and order semantics as before.
• Reduced collection time in synthetic large-CFG benchmark.

Code

• refactron/analysis/data_flow.py — _collect_nodes