fs: reject open()/read() on a directory (EISDIR)

open() now checks the target's inode type: opening a directory as a file
returns $FE (EISDIR) instead of a valid fd, so 'cat docs' no longer streams the
raw directory block as bytes. cat reports 'is a directory'; wc/head/save treat
any invalid fd (>3) as an open failure. (ls is unaffected - it uses the
structural opendir/list path, not open/getb.)

1 files changed, 1 insertions, 0 deletions

diff --git a/c/gbos.h b/c/gbos.h
index c93414f..2a99c0e 100644
--- a/c/gbos.h
+++ b/c/gbos.h
@@ -34,6 +34,7 @@ void          poweroff(void);   /* clean emulator shutdown (saves) */
 #define O_READ  0
 #define O_WRITE 1
 #define NOFD    0xFF
+#define EISDIR  0xFE     /* open() failed: path is a directory */
 
 /* process control (c/libc.s) */
 unsigned char fork(void);