feat: import cycle detection in ImportIntegrityVerifier

[omsherikar]

Context

ImportIntegrityVerifier documents deferred work:

Step 4 (cycle detection) deferred — requires project-wide import graph

(refactron/verification/checks/imports.py)

Impact

Transforms that introduce circular imports can still pass syntax + resolvability checks but break at runtime. Catching this in the verification engine materially increases confidence for autofix and --verify.

Suggested direction

• Build a bounded import graph for the project (or for the package subtree under project_root).
• On transform, diff new edges and detect simple cycles involving the changed module.
• Guardrails: timeouts, max files, cache graph between runs for performance.

Acceptance sketch

• Unit tests with minimal fake packages showing a cycle is reported with a clear blocking_reason.
• Document complexity limits in docstrings or config.

References

• refactron/verification/checks/imports.py (TODO)