Add VFS subsystem and initrd filesystem driver (AI)

VFS subsystem (vfs.c/h):
- Mount table with up to 16 mount points, longest-prefix path matching.
- File descriptor table (256 entries, fds 0-2 reserved for std streams).
- Path resolution walks mount table then delegates to filesystem's
  finddir() for each path component.
- Operations: open, close, read, write, seek, readdir, stat.
- Each filesystem driver provides a vfs_fs_ops_t with callbacks.

Initrd filesystem driver (initrd_fs.c/h):
- Read-only VFS driver backed by the CPIO ramdisk.
- Mounted at '/initrd' during boot.
- Zero-copy reads: file data points directly into the CPIO archive
  memory, no allocation or copying needed.
- Supports readdir (flat iteration) and finddir (name lookup).

Bug fix: resolve_path was overwriting file-specific fs_data (set by
finddir, e.g. pointer to CPIO file data) with the mount's fs_data
(NULL). Fixed to preserve fs_data from finddir.

Verified in QEMU: kernel reads /initrd/README via VFS and prints its
contents. Ring 3 user process continues to work.

README.md

@@ -51,8 +51,8 @@ Once a task is completed, it should be checked off.
 - [x] Create a VGA driver. On startup, some memory statistics should be displayed, as well as boot progress.
 - [x] Create subsystem for loading new processes in Ring 3.
 - [x] Update the build script to generate a ramdisk containing any applications to run. This initial ramdisk is in CPIO format.
-- [ ] Write a VFS subsystem.
-- [ ] Write a VFS driver that provides the contents of the CPIO initial ramdisk to the VFS layer.
+- [x] Write a VFS subsystem.
+- [x] Write a VFS driver that provides the contents of the CPIO initial ramdisk to the VFS layer.
 - [ ] Create a `hello-world` app. It should print `Hello, World` to its own stdout. The kernel should route this to Qemu and to the VGA dispaly. Ensure this work.
 - [ ] Implement the fork system call.
 - [ ] Implement environment variables. Apps should be able to modify this, and it should be copied to new forks of an app.