Add FAT32 driver, mount app, fix shell cd/path resolution

- FAT32 VFS driver (fat32.h, fat32.c): reads BPB, FAT table, cluster
  chains, directory entries (8.3 SFN), file read/write, mount via
  fat32_mount_device()
- mount app: writes 'device mountpoint' to /sys/vfs/mount to trigger
  FAT32 mount from userspace
- VFS sysfs mount namespace in kernel.c: handles mount requests via
  sysfs write to /sys/vfs/mount, delegates to FAT32 driver
- Shell cd: validates target directory exists before updating CWD,
  resolves relative paths (., .., components) to absolute paths
- Shell run_command: resolves ARG1 paths relative to CWD so apps
  like cat and ls receive absolute paths automatically
- Fixed FAT32 root directory access: use fs->root_cluster when
  VFS mount root node has inode=0

src/CMakeLists.txt

@@ -23,6 +23,7 @@ add_executable(kernel
     sysfs.c
     ide.c
     mbr.c
+    fat32.c
     env.c
     keyboard.c
     interrupts.S