fix(detector): detect exec string pointer replay for red 343

[prasannakotyal]

KernelGuard-Red-Submission: 343

Adds a narrow LAST_CALL_REPLAY pass for constant exec/builtins.exec strings that install custom_kernel and contain a data_ptr()-keyed output replay cache.

The detector requires:

• a constant string passed to exec or builtins.exec
• the exec payload to reference custom_kernel
• a data_ptr()-derived key
• a cache membership fast path returning cache[key]
• a later cache[key] = ... store in the exec payload

Local validation:

• UV_CACHE_DIR=/tmp/uvcache uv run python -m py_compile competition_state/kernelguard_mirror/kernelguard.py
• UV_CACHE_DIR=/tmp/uvcache uv run python kernelguard_bypasses/eval_blue_patch.py competition_state/kernelguard_mirror/kernelguard.py
• external accepted reds filtered locally: 85 and 343

Target red artifact: SarahLacard red 343, builtins.exec attribute call variant.

[github-actions]

KernelGuard Blue Evaluation

• Evaluation job: 171
• Blue submission: 143
• 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/25257403859

[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.