fix(detector): detect namedexpr state replay

[prasannakotyal]

KernelGuard-Red-Submission: 263

Adds a narrow LAST_CALL_REPLAY pass for assignment-expression aliases of replay state.

The detector requires:

• an entrypoint if test containing a named expression like (_x := _STATE)
• the else branch returning the alias
• the if body assigning the original state from a live input-derived expression
• the if body returning that same state

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
• target artifact /tmp/kg_external_reds/0263_SarahLacard_red-p-107.py is filtered as LAST_CALL_REPLAY

Target red artifact: SarahLacard red 263, assignment-expression guard variant.

[github-actions]

KernelGuard Blue Evaluation

• Evaluation job: 172
• Blue submission: 144
• 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/25257451653

[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.