Add public headers

Declarations for the string core, stdio, stdlib, unistd, errno, sys/types, and the raw syscall interface; the FILE layout stays private in src/internal.h.

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

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

include/stdlib.h

@@ -0,0 +1,38 @@
+/*
+ * stdlib.h — general utilities for nulsl-libc.
+ *
+ * The memory functions are ENOSYS stubs until the brk-based allocator
+ * lands; atoi and the exit paths are already real.
+ */
+
+#ifndef _NULSL_STDLIB_H
+#define _NULSL_STDLIB_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define EXIT_SUCCESS 0
+#define EXIT_FAILURE 1
+
+/* Stubs: brk()/mmap() allocator is on the roadmap. */
+void *malloc(size_t size);
+void *calloc(size_t nmemb, size_t size);
+void *realloc(void *ptr, size_t size);
+void free(void *ptr);
+
+/* Real. */
+void exit(int status);
+void abort(void);
+int atoi(const char *s);
+
+/* Stub. */
+long strtol(const char *s, char **endptr, int base);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NULSL_STDLIB_H */