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/bench_vlibc.c

@@ -0,0 +1,53 @@
+/*
+ * 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;
+}