F422 static, structural, and review checks

build LLVM 18 SymCC: PASS
build LLVM 17 SymCC: PASS
py_compile changed Python modules: PASS
ruff changed Python modules: PASS
git diff --check relevant sources and docs: PASS
diagram SVG render and visual inspection: PASS
diagram PNG dimensions: 1600x950
strict mutation battery: PASS
late prepare failure leaves ExecutionState unchanged: PASS
summary CFG exposes fallback and target: PASS
summary CFG creates no path-cover/CBC/CGS branch token: PASS
scalar live-out producer fail-closed: PASS
F422 LLVM17 focused: 2/2 PASS
F422 LLVM18 focused: 2/2 PASS
F416--F422 LLVM17 related: 16/16 PASS
F416--F422 LLVM18 related: 16/16 PASS
F416--F422 Python related: 35/35 PASS
complete Python: 1111 passed + 250 subtests PASS
complete LLVM17: 302 passed + 2 unsupported PASS
complete LLVM18: 303 passed + 1 unsupported PASS