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).

.clang-format

@@ -0,0 +1,37 @@
+# clang-format configuration for vlibc.
+# Keep in sync with STYLEGUIDE.md.
+
+BasedOnStyle: LLVM
+Language: Cpp
+
+IndentWidth: 4
+TabWidth: 4
+UseTab: Never
+ContinuationIndentWidth: 4
+ColumnLimit: 100
+
+BreakBeforeBraces: Custom
+BraceWrapping:
+  AfterCaseLabel: true
+  AfterClass: true
+  AfterControlStatement: Always
+  AfterEnum: true
+  AfterFunction: true
+  AfterNamespace: true
+  AfterStruct: true
+  AfterUnion: true
+  AfterExternBlock: false
+  BeforeCatch: true
+  BeforeElse: true
+  IndentBraces: false
+  SplitEmptyFunction: false
+AlwaysBreakAfterReturnType: TopLevel
+PointerAlignment: Right
+DerivePointerAlignment: false
+
+AlignAfterOpenBracket: Align
+AlignConsecutiveAssignments: false
+AlignConsecutiveDeclarations: false
+
+SortIncludes: CaseSensitive
+ReflowComments: false