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/Makefile.am

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+# vlibc — benchmark harnesses (musts/BENCHMARKING.md).
+
+AM_CPPFLAGS = -I$(top_srcdir)/include
+AM_CFLAGS = @VLIBC_CFLAGS@
+
+# Built only on demand (via `make bench`), so `make all` does not need the
+# library to be built first.
+EXTRA_PROGRAMS = bench_vlibc
+bench_vlibc_SOURCES = bench_vlibc.c
+
+# By default benchmarks link against vlibc itself. --with-libc=glibc links the
+# same harness against the system glibc; --with-libc=musl builds it with
+# musl-gcc for the musl reference.
+if BENCH_LINK_VLIBC
+bench_vlibc_LDADD = ../libvlibc.la
+else
+bench_vlibc_LDADD =
+endif
+
+if BENCH_LINK_MUSL
+CC = $(MUSL_CC)
+endif
+
+bench: bench_vlibc
+	./bench_vlibc
+
+.PHONY: bench

benchmarks/README.md

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+# 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.

benchmarks/bench_vlibc.c

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+/*
+ * Benchmark harness for vlibc (musts/BENCHMARKING.md).
+ *
+ * Every vlibc component is benchmarked against the software it replaces.
+ * This stub times vlibc_version() and is the skeleton that per-component
+ * benchmarks build on. Reconfigure with --with-libc=musl or --with-libc=glibc
+ * to link the same harness against a reference libc for comparison.
+ */
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <vlibc.h>
+
+#include <stdio.h>
+#include <time.h>
+
+#define ITERATIONS 100000000ULL
+
+int
+main(void)
+{
+    volatile const char *version = NULL;
+    struct timespec start;
+    struct timespec end;
+    double whole;
+    double frac;
+    double seconds;
+
+    if (clock_gettime(CLOCK_MONOTONIC, &start) != 0)
+    {
+        return 1;
+    }
+
+    for (unsigned long long i = 0; i < ITERATIONS; i++)
+    {
+        version = vlibc_version();
+    }
+
+    if (clock_gettime(CLOCK_MONOTONIC, &end) != 0)
+    {
+        return 1;
+    }
+
+    whole = (double)(end.tv_sec - start.tv_sec);
+    frac = (double)(end.tv_nsec - start.tv_nsec) / 1000000000.0;
+    seconds = whole + frac;
+
+    printf("vlibc_version() x %llu: %.3f s (%.2f ns/call), version=%s\n", ITERATIONS, seconds,
+           seconds * 1000000000.0 / ITERATIONS, (const char *)version);
+
+    return 0;
+}