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/overview.md

@@ -0,0 +1,46 @@
+# Overview
+
+vlibc is Vox's replacement for glibc: a C and POSIX library that is modern,
+standard-conforming, and fast. The behavior of the software and its
+differences from glibc are documented here so they can be understood without
+reading optimized source.
+
+## Goals
+
+- **Replace glibc** for the programs that want a lighter, faster, and cleaner
+  libc, while retaining high compatibility.
+- **Be benchmarkable**: every component is measured against glibc (and musl);
+  a component slower than what it replaces is considered failing
+  (`musts/BENCHMARKING.md`).
+- **Static-linking first**: fully static linking is a hard requirement for
+  every profile except `spoof`.
+- **Honor user flags**: `CFLAGS` and `LDFLAGS` are respected; the build uses
+  exactly one compiler (GCC) with no fallback.
+
+## Compatibility profiles
+
+See `docs/compatibility.md` for the five profiles (`onlyposix`, `muslmimic`,
+`muslext`, `spoof`, `vlibc`) and their differences from glibc.
+
+## Install methods
+
+See `docs/install.md` for the `alongside` and `overwrite` install methods and
+the `vlibc-gcc` / `vlibc-clang` drivers.
+
+## Compiler intent
+
+Static linking pulls a whole library into every binary, which is expensive in
+size. To offset this, the public API declares *intent* to the compiler via
+attributes such as `__attribute__((const))` and `__attribute__((pure))`. When
+the compiler knows a call has no side effects and a predictable result, it can
+fold or eliminate it during optimization, shrinking the final binary even when
+vlibc is statically linked in full.
+
+This is a deliberate design constraint: every public function carries the
+tightest correct intent attribute (see `STYLEGUIDE.md`).
+
+## Status
+
+This repository is currently a stub — the layout, build system, documentation,
+and a minimal public API (`vlibc_version()`) are in place. Individual library
+components are added incrementally, each with benchmarks.