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/string.h

@@ -0,0 +1,102 @@
+#ifndef VLIBC_STRING_H
+#define VLIBC_STRING_H
+
+/*
+ * vlibc — <string.h>.
+ *
+ * String and memory functions, gated by the active compatibility profile
+ * (see include/vlibc/features.h). Levels are cumulative:
+ *
+ *   Level 1 (onlyposix): ISO C core — memcpy, memmove, memset, strlen, strcmp.
+ *   Level 2 (muslmimic): BSD extensions — strlcpy, strlcat.
+ *   Level 3 (muslext):   GNU extensions — strcasestr.
+ *
+ * This header includes <vlibc/features.h> itself, so the gates below always
+ * see the configured VLIBC_LEVEL even when the caller included no vlibc
+ * header first, and <stddef.h> for size_t.
+ */
+
+#include <vlibc/features.h>
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Level 1: ISO C core (always present). */
+
+/*
+ * Return the length of s, excluding the terminating NUL.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) size_t
+strlen(const char *s);
+
+/*
+ * Compare the strings lhs and rhs; return negative, zero, or positive when
+ * lhs is less than, equal to, or greater than rhs.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) int
+strcmp(const char *lhs, const char *rhs);
+
+/*
+ * Copy n bytes from src to dst. The regions must not overlap (restrict).
+ * No intent attribute: it writes memory.
+ */
+void *
+memcpy(void *restrict dst, const void *restrict src, size_t n);
+
+/*
+ * Copy n bytes from src to dst. The regions may overlap, so the parameters
+ * are deliberately not restrict.
+ * No intent attribute: it writes memory.
+ */
+void *
+memmove(void *dst, const void *src, size_t n);
+
+/*
+ * Fill n bytes at dst with c (converted to unsigned char).
+ * No intent attribute: it writes memory.
+ */
+void *
+memset(void *dst, int c, size_t n);
+
+#if VLIBC_LEVEL >= 2
+/* Level 2 (muslmimic): BSD extensions. */
+
+/*
+ * Copy at most size - 1 bytes from src to dst and NUL-terminate the result;
+ * return strlen(src). No NUL is written when size is 0.
+ * No intent attribute: it writes memory.
+ */
+size_t
+strlcpy(char *dst, const char *src, size_t size);
+
+/*
+ * Append src to dst, NUL-terminating within size bytes; return the length of
+ * the string that would have been created without truncation.
+ * No intent attribute: it writes memory.
+ */
+size_t
+strlcat(char *dst, const char *src, size_t size);
+#endif /* VLIBC_LEVEL >= 2 */
+
+#if VLIBC_LEVEL >= 3
+/* Level 3 (muslext): GNU extensions. */
+
+/*
+ * Return a pointer to the first case-insensitive (ASCII fold) occurrence of
+ * needle in haystack, or NULL if absent; an empty needle matches haystack.
+ * pure: reads memory, no side effects.
+ */
+__attribute__((pure)) char *
+strcasestr(const char *haystack, const char *needle);
+#endif /* VLIBC_LEVEL >= 3 */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* VLIBC_STRING_H */