Refactor AST traversal for improved performance and accuracy

[akanthed]

• Introduced a file indexing mechanism to reduce redundant AST walks across multiple rules, significantly improving scan performance.
• Replaced direct descendant traversal with indexed methods for retrieving call expressions and function nodes.
• Updated various rules to utilize the new indexing functions, ensuring consistent behavior and performance gains.
• Added tests to validate the correctness of the new indexing methods against traditional traversal methods.
• Introduced new test fixtures to cover edge cases and ensure robustness of the scanning process.
• Enhanced Python AST parsing to correctly identify and handle calls, attributes, and other constructs, improving vulnerability detection.

What and why

Detection logic changes (delete this section if not applicable)

• npm run regression was run and every proven/likely finding it printed was reviewed against its source line
• A new/updated rule has fixtures in both test-fixtures/vulnerable/ and test-fixtures/safe/
• src/scanner/catalog.ts and src/scanner/explainer.ts both have an entry for any new rule ID
• test/corpus.test.js's EXPECTED_VULNERABLE was updated for any new rule

CLI changes (delete this section if not applicable)

• test/cli.test.js has a new or updated case exercising the built binary (src/cli.ts changes aren't covered by any other test file)

Checklist

• npm run build && npm test passes
• Before/after behavior is described above, with a concrete example if the change affects scan output

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