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/compatibility.md

@@ -0,0 +1,30 @@
+# Compatibility
+
+vlibc offers five compatibility profiles, selected at configure time. They are
+mutually exclusive; the default is `vlibc`.
+
+| Level | Flag                 | Profile     | What it provides                                        |
+|-------|----------------------|-------------|---------------------------------------------------------|
+| 1     | `--enable-onlyposix` | `onlyposix` | Pure POSIX, nothing more — the lightest possible build. |
+| 2     | `--enable-muslmimic` | `muslmimic` | musl-like, still light, with musl-compatible features.  |
+| 3     | `--enable-muslext`   | `muslext`   | An extension of musl, adding more.                      |
+| 4     | `--enable-spoof`     | `spoof`     | A glibc replica: emulates glibc for drop-in compatibility, higher than `muslext` or `vlibc` full. |
+| 5     | *(default)*          | `vlibc`     | glibc-extended: glibc minus its baggage, plus extended standard features. High (but not spoof-level) glibc compatibility. |
+
+## Differences from glibc
+
+- **No legacy baggage.** vlibc targets modern, standard-conforming behavior and
+  drops glibc's deprecated and non-standard behaviors unless a profile
+  explicitly restores them.
+- **Extended standard features.** The `vlibc` profile adds extensions on top of
+  the C and POSIX standards that glibc does not provide.
+- **Spoofing is opt-in.** The `--enable-spoof` profile re-enables the
+  long-standing legacy behaviors that scripts rely on, for drop-in
+  compatibility with existing binaries and build systems.
+
+## Full static linking
+
+Every profile except `spoof` must be able to link fully statically. The
+`spoof` profile is exempt: its glibc-emulation layer depends on dynamic
+facilities (e.g. `dlopen`-based compatibility shims) that a static build cannot
+provide.