feat(profile): add stddef.h and profile-gated string.h

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <[email protected]>

include/stddef.h

@@ -0,0 +1,45 @@
+#ifndef VLIBC_STDDEF_H
+#define VLIBC_STDDEF_H
+
+/*
+ * vlibc — <stddef.h>.
+ *
+ * Self-contained definitions of the common C types and macros (C23). Every
+ * definition here is derived from compiler builtins, so this header never
+ * borrows from a system header.
+ *
+ * The compatibility profile is exposed by include/vlibc/features.h; it is
+ * included here so consumers always see the configured VLIBC_LEVEL.
+ */
+
+#include <vlibc/features.h>
+
+/* Integer type of the result of the sizeof operator. */
+typedef __SIZE_TYPE__ size_t;
+
+/* Signed integer type of the difference of two pointers. */
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+
+/* Wide character type. */
+typedef __WCHAR_TYPE__ wchar_t;
+
+/* Type of the null pointer constant nullptr (C23). */
+typedef typeof(nullptr) nullptr_t;
+
+/* Null pointer constant. */
+#define NULL ((void *)0)
+
+/* Offset in bytes of a member from the start of its enclosing object. */
+#define offsetof(type, m) __builtin_offsetof(type, m)
+
+/*
+ * An object type whose alignment is as great as that of any supported
+ * fundamental type; suitable as the storage type for aligned allocation.
+ */
+typedef struct
+{
+    long long vll;
+    long double vld;
+} max_align_t;
+
+#endif /* VLIBC_STDDEF_H */