Add vlibc scaffold, build system, and documentation
Establish the repository layout per Vox guidelines (layouts/C.md, building/C.md): Autotools build (GCC-only, C23), five compatibility profiles (--enable-onlyposix/--enable-muslmimic/--enable-muslext/ --enable-spoof, default vlibc), alongside/overwrite install methods, vlibc-gcc/vlibc-clang drivers, static-linking requirement (except spoof), benchmark harness, and tooling (.clang-format/.clang-tidy/ .clangd + compile_commands.json).
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