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/sys/syscall.h

@@ -0,0 +1,51 @@
+/*
+ * sys/syscall.h — the raw syscall interface.
+ *
+ * syscall() is the ONLY place in nulsl-libc that talks the kernel ABI.
+ * Everything above it (read, write, exit, ...) is a wrapper.
+ *
+ * SYS_* constants below are the x86_64 numbers for the syscalls the libc
+ * itself wraps or its tests use. They are #ifndef-guarded so kernel UAPI
+ * headers can coexist. Extend the table as needed (docs/syscalls.md).
+ */
+
+#ifndef _NULSL_SYS_SYSCALL_H
+#define _NULSL_SYS_SYSCALL_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+long syscall(long number, ...);
+
+/* x86_64 syscall numbers (Linux ABI). */
+#ifndef SYS_read
+#define SYS_read 0
+#endif
+#ifndef SYS_write
+#define SYS_write 1
+#endif
+#ifndef SYS_open
+#define SYS_open 2
+#endif
+#ifndef SYS_close
+#define SYS_close 3
+#endif
+#ifndef SYS_getpid
+#define SYS_getpid 39
+#endif
+#ifndef SYS_exit
+#define SYS_exit 60
+#endif
+#ifndef SYS_unlink
+#define SYS_unlink 87
+#endif
+#ifndef SYS_clock_gettime
+#define SYS_clock_gettime 228
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NULSL_SYS_SYSCALL_H */

include/sys/types.h

@@ -0,0 +1,25 @@
+/*
+ * sys/types.h — primitive system types for nulsl-libc.
+ */
+
+#ifndef _NULSL_SYS_TYPES_H
+#define _NULSL_SYS_TYPES_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef long ssize_t;
+typedef long off_t;
+typedef long pid_t;
+typedef unsigned int uid_t;
+typedef unsigned int gid_t;
+typedef unsigned long mode_t;
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NULSL_SYS_TYPES_H */