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).

docs/install.md

@@ -0,0 +1,43 @@
+# Install methods
+
+vlibc has two install methods, selected with `--with-install=`.
+
+## Alongside (default)
+
+```sh
+./autogen.sh --with-install=alongside
+make
+make install
+```
+
+- Does **not** replace the system libc.
+- Installs headers and libraries under a vlibc-specific tree:
+  `$prefix/lib/vlibc/include` and `$prefix/lib/vlibc/lib`.
+- Consuming projects use the shipped drivers:
+
+```sh
+vlibc-gcc  -o app app.c    # GCC backend
+vlibc-clang -o app app.c   # Clang backend
+```
+
+## Overwrite
+
+```sh
+./autogen.sh --with-install=overwrite
+make
+make install
+```
+
+- Replaces the system libc in place: headers go to `$prefix/include`, libraries
+  to `$prefix/lib`.
+- The shipped drivers then use the system include/lib paths directly.
+
+## Compiler drivers
+
+`vlibc-gcc` and `vlibc-clang` are thin wrappers generated by `configure`. They
+add vlibc's include and library paths (according to the install method) and
+delegate to `gcc` / `clang`.
+
+> **Note:** the drivers are currently stubs. Full sysroot handling for
+> `overwrite` installs and cross-compilation, and the `-static`/`-lvlibc`
+> wiring, are added as the library's ABI matures.