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, 7 insertions, 2 deletions

diff --git a/src/proc.asm b/src/proc.asm
index cd79160..c22d40d 100644
--- a/src/proc.asm
+++ b/src/proc.asm
@@ -226,7 +226,9 @@ ProcCreate::
     ld [hl+], a                 ; PROC_STDIN  = console
     ld [hl+], a                 ; PROC_STDOUT = console
     ld a, $FF
-    ld [hl], a                  ; PROC_PROG   = none
+    ld [hl+], a                 ; PROC_PROG   = none
+    ld a, ROOT_INO
+    ld [hl], a                  ; PROC_CWD    = root
 
     ; build the initial stack frame in the task's RAM bank
     ld a, [wTmpEntry]
@@ -458,7 +460,10 @@ sys_fork::
     ld [hl+], a                 ; child STDOUT = parent STDOUT
     inc de
     ld a, [de]
-    ld [hl], a                  ; child PROC_PROG = parent PROC_PROG
+    ld [hl+], a                 ; child PROC_PROG = parent PROC_PROG
+    inc de
+    ld a, [de]
+    ld [hl], a                  ; child PROC_CWD = parent PROC_CWD
 
     ; ---- return to the parent with child pid ----
     ld a, [wForkUserSP]