refactor: decouple DevFS from TTY/PTY drivers via device registration API

refactor: decouple DevFS from TTY/PTY drivers via device registration API

Added devfs_register_device() API so device drivers register their own
fs_node_t with DevFS. DevFS is now a generic device registry that
dispatches through function pointers — it no longer includes tty.h or
pty.h and has zero knowledge of TTY/PTY internals.

- tty.c: registers /dev/console and /dev/tty with VFS-compatible wrappers
- pty.c: registers /dev/ptmx and /dev/pts (with finddir/readdir moved
  from devfs.c)
- devfs.c: only owns built-in devices (null, zero, random, urandom);
  all other devices come from the registry

This enables any future driver to register device nodes without
modifying devfs.c.