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

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+# 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.

docs/install.md

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+# 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.

docs/overview.md

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+# 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.