fix(detector): detect getitem store replay

[prasannakotyal]

Summary

Targets KG red #196, where custom_kernel stores the first input-derived output inside an object, then replays it through that object's custom __getitem__ path.

This adds a narrow GETITEM_STORE_REPLAY detector. It requires:

• a local class defining __getitem__
• a module-level instance of that class
• an entrypoint mutating one of that instance's attribute containers from input-derived data
• an input-independent guard over the same instance
• a call-free replay body returning a subscript of that instance

Method reader replay and bool-method replay stay out of scope.

KernelGuard-Red-Submission: 196

Local verification

• red_82_getitem_store_replay -> GETITEM_STORE_REPLAY, should_filter=True
• red_83_method_store_replay stayed valid
• red_115_method_reader_replay stayed valid
• red_81_bool_method_replay stayed valid
• clean fixtures in eval_blue_patch.py stayed valid
• uv run python -m py_compile kernelguard.py

[github-actions]

KernelGuard Blue Evaluation

• Evaluation job: 100
• Blue submission: 72
• 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/25244097878

[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.