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).

benchmarks/README.md

@@ -0,0 +1,33 @@
+# Benchmarking
+
+Every vlibc component is benchmarked against the software it replaces, per
+`musts/BENCHMARKING.md`. A component that is slower or worse at its task than
+the software it replaces is considered failing.
+
+## Layout
+
+- Each benchmark harness is a standalone C program in this directory, named
+  `bench_<component>.c`.
+- `bench_vlibc.c` is the skeleton that per-component benchmarks build on.
+
+## Running
+
+```sh
+./autogen.sh                  # configure (default: link against vlibc)
+make bench                    # build and run the harnesses
+
+# Reference comparison against the libc vlibc replaces:
+./autogen.sh --with-libc=glibc
+make bench
+
+# musl reference (requires musl-gcc):
+./autogen.sh --with-libc=musl
+make bench
+```
+
+## Adding a benchmark
+
+1. Add `bench_<component>.c` here and list it in `benchmarks/Makefile.am`.
+2. Measure the component against the equivalent glibc (and musl) call.
+3. Report wall-clock time, throughput, and any binary-size difference.
+4. If vlibc is slower, the benchmark fails — fix the implementation.