feat(string): add GNU strcasestr at level 3

Case-insensitive ASCII-fold substring search, locale-free, no
allocation.

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

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

src/string/strcasestr.c

@@ -0,0 +1,59 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <string.h>
+
+/*
+ * Fold an ASCII uppercase letter to lowercase; every other character
+ * (including NUL) passes through unchanged. Kept local and locale-free:
+ * strcasestr must never depend on ctype tables or the active locale.
+ */
+static inline char
+fold_ascii(char c)
+{
+    if (c >= 'A' && c <= 'Z')
+    {
+        return (char)(c + ('a' - 'A'));
+    }
+
+    return c;
+}
+
+/*
+ * Return a pointer to the first case-insensitive (ASCII fold) occurrence of
+ * needle in haystack, or NULL if absent. An empty needle matches haystack.
+ *
+ * Naive two-pointer scan: for each position in haystack, compare folded
+ * characters until the needle is exhausted (match) or a mismatch occurs
+ * (advance). The inner loop stops at the NUL terminator of either string,
+ * so the comparison never reads past the end of haystack when needle is
+ * longer than the remaining tail.
+ */
+char *
+strcasestr(const char *haystack, const char *needle) // NOLINT(bugprone-easily-swappable-parameters)
+{
+    if (*needle == '\0')
+    {
+        return (char *)haystack;
+    }
+
+    for (; *haystack != '\0'; haystack++)
+    {
+        const char *h = haystack;
+        const char *n = needle;
+
+        while (*n != '\0' && fold_ascii(*h) == fold_ascii(*n))
+        {
+            h++;
+            n++;
+        }
+
+        if (*n == '\0')
+        {
+            return (char *)haystack;
+        }
+    }
+
+    return NULL;
+}