iter 23.2.1: runtime/str.c with ail_str_eq + unconditional link

runtime/str.c

@@ -0,0 +1,26 @@
+/* AILang Str runtime primitives.
+ *
+ * Provides ABI-stable string operations against the existing
+ * constant-Str layout (NUL-terminated byte buffer, passed as
+ * `ptr` in LLVM IR). Lives separately from runtime/rc.c so that
+ * a future heap-Str ABI milestone can swap the implementations
+ * without touching the RC runtime.
+ *
+ * Linked unconditionally by `ail build` (Gc, Bump, Rc) so that
+ * any program that monomorphises `eq__Str` (auto-loaded from
+ * the prelude) finds the symbol. Programs that never call
+ * `eq` on Str leave @ail_str_eq as a dead-stripped no-op at
+ * link time under clang -O2.
+ */
+
+#include <stdbool.h>
+#include <string.h>
+
+/* Byte-equality on NUL-terminated strings. Mirrors strcmp(...) == 0
+ * but exposed under a stable AILang-namespaced ABI so the codegen
+ * caller does not assume libc's strcmp shape. Heap-Str ABI milestone
+ * will replace the body with length-prefixed comparison without
+ * changing the signature. */
+bool ail_str_eq(const char *a, const char *b) {
+    return strcmp(a, b) == 0;
+}