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).

.clangd

@@ -0,0 +1,19 @@
+# clangd language-server configuration for vlibc.
+# Provides fallback flags so the LSP reports errors on valid code even before
+# compile_commands.json is regenerated (see layouts/C.md).
+
+CompileFlags:
+  CompilationDatabase: .
+  Add:
+    - -std=c23
+    - -Iinclude
+
+Diagnostics:
+  UnusedIncludes: Strict
+  ClangTidy:
+    Add:
+      - bugprone-*
+      - clang-analyzer-*
+
+Index:
+  Background: Build