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).

.clang-tidy

@@ -0,0 +1,19 @@
+# clang-tidy configuration for vlibc.
+# Keep in sync with STYLEGUIDE.md.
+
+Checks: >
+  bugprone-*,
+  clang-analyzer-*,
+  misc-*,
+  performance-*,
+  portability-*,
+  readability-*,
+  -readability-magic-numbers,
+  -readability-identifier-length,
+  -readability-function-cognitive-complexity,
+  -misc-include-cleaner
+
+WarningsAsErrors: 'bugprone-*,clang-analyzer-*'
+HeaderFilterRegex: '^include/'
+
+FormatStyle: file