fix(detector): detect mappingproxy replay

[prasannakotyal]

Summary

Targets KG red #165, where custom_kernel reads replay state through a MappingProxyType proxy while mutating the proxy's backing dict from the current input.

This adds a narrow MAPPINGPROXY_REPLAY detector. It requires:

• a module-level dict backing store
• a module-level MappingProxyType(backing) proxy
• an entrypoint assignment to the backing dict from an input-derived call
• an input-independent guard over the proxy
• a call-free replay body returning a subscript of that same proxy

WeakKeyDictionary, OrderedDict, and unrelated mapping state stay out of scope.

KernelGuard-Red-Submission: 165

Local verification

• red_46_mappingproxy_replay -> MAPPINGPROXY_REPLAY, should_filter=True
• red_204_mappingproxy_tuple_key_replay -> MAPPINGPROXY_REPLAY, should_filter=True
• red_37_weakkeydict_replay stayed valid
• red_44_ordered_dict_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: 72
• Blue submission: 52
• 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/25238507162

[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.