F421 static and structural checks

build LLVM 18 SymCC: PASS
build LLVM 17 SymCC: PASS
py_compile changed Python modules: PASS
ruff check changed Python modules: PASS
git diff --check relevant sources and docs: PASS
diagram render and visual inspection: PASS
diagram PNG dimensions: 2400x1350
focused fixture files: 2
focused RUN commands: 20 (13 main, 7 fallback)
v11 schema references in producer/strict consumer/checker: 34
F421 LLVM17 focused: 2/2 PASS
F421 LLVM18 focused: 2/2 PASS
F416--F421 LLVM17 related: 14/14 PASS
F416--F421 LLVM18 related: 14/14 PASS
F421 Python targeted: 5/5 PASS
F416--F421 Python related: 32/32 PASS