perf: lazy libcst load and SyntaxVerifier hot-path tweaks

[omsherikar]

Summary

SyntaxVerifier.verify does import libcst inside the method on every invocation. That adds import overhead per fix/verify cycle even when short-circuiting could be optimized. Also, dangerous-call and import-count logic walks ASTs that could share a single ast.parse if the engine provides it (see related issue on shared AST).

Suggested direction

• Move libcst to module-level lazy import pattern (e.g. import on first use cached in a module global) or top-level import if dependency is always present for verification.
• After shared-parse work lands, reuse one ast for _find_dangerous_calls / _count_imports where it reduces walks.

Acceptance

• Reduced per-call import cost in profiles or micro-benchmarks.
• Behavior unchanged for CST round-trip failures and dangerous-call detection.

Code

• refactron/verification/checks/syntax.py