fix(detector): GLOBAL_SENTINEL_REUSE — catch module-level None sentinel replay

[SarahLacard]

KernelGuard-Red-Submission: 30

Catches the null-sentinel pattern (module-level _x = None with global declaration, if _x is None guard) used for result caching in entrypoint. This technique was scoring classification: valid, should_filter: false.

Target catches exactly red #30 (null-sentinel pointer compare replay). TP/FP: clean. Surgicalness: 1.0.

[github-actions]

KernelGuard Blue Evaluation

• Evaluation job: 95
• Blue submission: 38
• Result: passed
• Target red submission caught: True
• Validation suite: TP 20/20, FP 20/20
• Surgicalness score: 1.0
• Workflow run: https://github.com/SinatrasC/kernelguard/actions/runs/25243945571

[SarahLacard]

Closing: Detector GLOBAL_SENTINEL_REUSE checks 'is None' but RED #30 uses 'is not None' + BoolOp — doesn't catch the target red

[SinatrasC]

Thanks for the KernelGuard Flywheel Campaign contribution. We are not merging this narrow variant separately because the consolidated rule-family implementation in #273 is the merge path for this detector area.