feat(headers): stddef/stdint/stdbool/limits/float/assert + features wiring

include/stdckdint.h

@@ -0,0 +1,23 @@
+#ifndef VLIBC_STDCKDINT_H
+#define VLIBC_STDCKDINT_H
+
+/*
+ * vlibc — <stdckdint.h>.
+ *
+ * Checked integer arithmetic (C23). Each macro computes the operation and
+ * stores the result through *result; it returns true when the mathematical
+ * result is not representable in the result type, in which case *result is
+ * left unmodified. The implementation defers to the GCC
+ * __builtin_*_overflow family, which is type-generic across every integer
+ * type, matching the standard's semantics.
+ *
+ * This header is ISO C core and is present in every profile.
+ */
+
+#include <vlibc/features.h>
+
+#define ckd_add(result, a, b) __builtin_add_overflow((a), (b), (result))
+#define ckd_sub(result, a, b) __builtin_sub_overflow((a), (b), (result))
+#define ckd_mul(result, a, b) __builtin_mul_overflow((a), (b), (result))
+
+#endif /* VLIBC_STDCKDINT_H */