iter 23.3.1: runtime ail_str_compare + IR header declaration

runtime/str.c

@@ -24,3 +24,17 @@
 bool ail_str_eq(const char *a, const char *b) {
     return strcmp(a, b) == 0;
 }
+
+/* Three-way comparison of NUL-terminated strings. Normalises libc
+ * strcmp's signed-int output to exactly -1 / 0 / +1 so the codegen
+ * caller can branch on `icmp slt i32 result, 0` and
+ * `icmp eq i32 result, 0` against constants directly without
+ * worrying about strcmp implementations that return values outside
+ * {-1, 0, +1}. Heap-Str ABI milestone will replace the body with
+ * length-prefixed comparison without changing the signature. */
+int ail_str_compare(const char *a, const char *b) {
+    int r = strcmp(a, b);
+    if (r < 0) return -1;
+    if (r > 0) return 1;
+    return 0;
+}