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).

include/vlibc.h

@@ -0,0 +1,45 @@
+#ifndef VLIBC_H
+#define VLIBC_H
+
+/*
+ * vlibc — public API.
+ *
+ * vlibc is Vox's modern replacement for glibc. This header is the public entry
+ * point for vlibc's own extensions; the C and POSIX standard headers
+ * (<stdio.h>, <string.h>, ...) are provided separately and are gated by the
+ * active compatibility profile.
+ *
+ * Compatibility profiles (build-time; see configure.ac and docs/compatibility.md):
+ *   1  onlyposix   pure POSIX, nothing more
+ *   2  muslmimic   musl-like, light
+ *   3  muslext     musl-extended
+ *   4  spoof       glibc replica (drop-in compatibility)
+ *   5  vlibc       glibc-extended, the default
+ */
+
+#define VLIBC_VERSION_MAJOR 0
+#define VLIBC_VERSION_MINOR 1
+#define VLIBC_VERSION_PATCH 0
+
+#define VLIBC_VERSION_STRING "0.1.0"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * Return the vlibc version string ("MAJOR.MINOR.PATCH").
+ *
+ * Declared with __attribute__((const)): the result is a compile-time constant
+ * and the call has no observable side effects. Declaring this intent lets the
+ * compiler fold and eliminate the call, which keeps fully statically linked
+ * binaries small (see docs/overview.md, "Compiler intent").
+ */
+__attribute__((const)) const char *
+vlibc_version(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* VLIBC_H */