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).

thirdparty/README.md

@@ -0,0 +1,14 @@
+# Third-party
+
+Third-party libraries are vendored here, per `layouts/C.md`.
+
+## Policy
+
+- Vendoring is required to prevent dependency hell: vlibc must build from a
+  checked-out tree with no external downloads.
+- Every vendored dependency records its upstream source, version, and license
+  in a `README` (or `UPSTREAM`) file alongside its sources.
+- Vendored code is patched in-tree where necessary; patches are documented.
+
+This directory is intentionally empty for now — the current stub has no
+third-party dependencies.