perf: index sinks/sources before ast.walk in taint analysis

[omsherikar]

Summary

TaintAnalyzer._check_sink calls ast.walk(stmt) for every statement and then performs name resolution and taint checks. Combined with repeated _is_expression_tainted recursion, this creates high overhead in large functions.

Suggested direction

• Build a lightweight per-statement call index (or pre-filter call-containing statements) before full traversal.
• Memoize expression taint checks within a statement/context to avoid repeated recursion on identical subtrees.
• Keep vulnerability output and de-dup semantics unchanged.

Acceptance

• No regression in detected vulnerabilities on current tests.
• Measurable reduction in taint-analysis runtime on larger fixture(s).

Code

• refactron/analysis/taint.py — _check_sink, _is_expression_tainted