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/unistd.h

@@ -0,0 +1,38 @@
+/*
+ * unistd.h — POSIX system call wrappers for nulsl-libc.
+ *
+ * These are thin, real wrappers around the raw syscall() interface — see
+ * docs/syscalls.md. The kernel is the API; everything else is sugar.
+ */
+
+#ifndef _NULSL_UNISTD_H
+#define _NULSL_UNISTD_H
+
+#include <sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define STDIN_FILENO  0
+#define STDOUT_FILENO 1
+#define STDERR_FILENO 2
+
+/* Real syscall wrappers. */
+ssize_t read(int fd, void *buf, size_t count);
+ssize_t write(int fd, const void *buf, size_t count);
+int close(int fd);
+pid_t getpid(void);
+void _exit(int status) __attribute__((noreturn));
+
+/* Stub. */
+int unlink(const char *path);
+
+/* Raw syscall entry point (also available here for convenience). */
+long syscall(long number, ...);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NULSL_UNISTD_H */