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/stdio.h

@@ -0,0 +1,46 @@
+/*
+ * stdio.h — standard I/O for nulsl-libc.
+ *
+ * Minimal for now: unbuffered writes to a file descriptor, plus ENOSYS
+ * stubs for the buffered/formatting machinery that is still to come.
+ * A FILE is just a file descriptor until we grow a real buffering layer.
+ */
+
+#ifndef _NULSL_STDIO_H
+#define _NULSL_STDIO_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define EOF (-1)
+
+#define SEEK_SET 0
+#define SEEK_CUR 1
+#define SEEK_END 2
+
+typedef struct nulsl_file FILE;
+
+extern FILE *stdin;
+extern FILE *stdout;
+extern FILE *stderr;
+
+/* Real, unbuffered. */
+int puts(const char *s);
+int putchar(int c);
+
+/* Stubs (see docs/architecture.md for the roadmap). */
+int printf(const char *fmt, ...);
+FILE *fopen(const char *path, const char *mode);
+int fclose(FILE *f);
+size_t fread(void *ptr, size_t size, size_t nmemb, FILE *f);
+size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *f);
+int fflush(FILE *f);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _NULSL_STDIO_H */