fs rewrite stage 3: nested directories, paths, cwd (cd/mkdir/ls)

- directories generalized: dir_find/dir_add/dir_remove take any dir inode;
  every dir carries '.'/'..'. Path resolution (resolve / resolve_parent) walks
  absolute or cwd-relative paths; open/mkdir/chdir/remove/opendir take paths.
- per-process cwd: PROC_CWD in the PCB (root by default, inherited on fork);
  sys_chdir sets it, sys_opendir points ls at any directory.
- shell: 'cd' builtin + cwd-aware prompt; 'mkdir'/'ls <dir>' tools; 'exit'/'quit'
  and EOF call poweroff() (clean shutdown via the $ED opcode -> reliable save).
- three HL/buffer-clobber bugs fixed along the way: resolve didn't preserve the
  path cursor across dir_find; cur_cwd clobbered HL; resolve_parent set the final
  name before resolve() overwrote wFsNameBuf (now copied after).
- verified: nested mkdir/cd/save, multi-level paths (cat docs/sub/b), and the
  whole tree persists across a reboot.
- README: document directories + the poweroff opcode.

1 files changed, 25 insertions, 0 deletions

diff --git a/c/libc.s b/c/libc.s
index c245bbd..d499521 100644
--- a/c/libc.s
+++ b/c/libc.s
@@ -221,3 +221,28 @@ _bpeek2:
 	rst	#0x30
 	ret
 
+
+	.globl _mkdir, _chdir
+	;; unsigned char mkdir(const char *path /*DE*/) -> A (0 ok / $FF)
+_mkdir:
+	ld	c, #25			; SYS_MKDIR
+	rst	#0x30
+	ret
+	;; unsigned char chdir(const char *path /*DE*/) -> A (0 ok / $FF)
+_chdir:
+	ld	c, #26			; SYS_CHDIR
+	rst	#0x30
+	ret
+
+	.globl _opendir
+	;; unsigned char opendir(const char *path /*DE*/) -> A (0 ok / $FF)
+_opendir:
+	ld	c, #27			; SYS_OPENDIR
+	rst	#0x30
+	ret
+
+	.globl _poweroff
+	;; void poweroff(void) - execute the emulator's clean-exit opcode ($ED)
+_poweroff:
+	.db	0xED
+1$:	jr	1$