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, 17 insertions, 6 deletions

diff --git a/README.md b/README.md
index 74f3608..a968e4a 100644
--- a/README.md
+++ b/README.md
@@ -304,9 +304,12 @@ blocks 2-3  inode table  (32 inodes x 16 B: type, size, 8 direct block ptrs)
 blocks 4..  data blocks
 ```
 
-- **Inodes + a root directory.** A directory is a file of 16-byte entries
-  (inode# + name). Files are up to **2 KiB** (8 direct blocks); up to **16**
-  entries in the root, **32** inodes.
+- **Inodes + nested directories.** A directory is a file of 16-byte entries
+  (inode# + name), each carrying `.`/`..`. Files are up to **2 KiB** (8 direct
+  blocks); **32** inodes, 16 entries per directory.
+- **Paths + per-process cwd.** `open`/`mkdir`/`chdir`/`remove`/`ls` take paths
+  (absolute `/a/b` or relative `a/b`); each process has a cwd inode (`PROC_CWD`,
+  inherited on fork). Shell builtins: `cd`, and a cwd-aware prompt.
 - **Bounce buffer.** Only `read_block`/`write_block` touch cart-RAM banking;
   everything else works on WRAM buffers. The bitmap + inode table are cached in
   WRAMX; data/dir blocks stream through a one-block cache.
@@ -320,8 +323,15 @@ $ save notes
 gbos has a real filesystem now
 $ cat notes
 gbos has a real filesystem now
-$ ls
-notes
+$ mkdir docs
+$ cd docs
+/docs$ save note
+hi
+/docs$ cd /
+$ cat docs/note
+hi
+$ ls docs
+note
 $ wc notes                 # survives a reboot
 1 5 31
 ```
@@ -347,7 +357,8 @@ $ wc notes                 # survives a reboot
   - [x] **stage 1**: block device (`blk.asm`) via a WRAM bounce buffer
   - [x] **stage 2**: inodes + root directory; variable-size files (<=2 KiB),
     bitmap allocators; 16 files / 32 inodes; persists across reboots
-  - [ ] stage 3: nested directories, path resolution, per-process cwd, `cd`/`mkdir`
+  - [x] **stage 3**: nested directories, path resolution, per-process cwd,
+    `cd`/`mkdir`/`ls <dir>` — the whole tree persists across reboots
 - [ ] Preemptive scheduling: real context save in `TimerISR` → `hSwitchTo`
 - [ ] Use the `$D000-$DFFF` u-area (SVBK) for per-process kernel state / kstack
 - [ ] `brk`/heap allocator inside the task bank (heap up, stack down, collision = ENOMEM)