feat: /dev/kbd raw scancode device for game input (DOOM)

feat: /dev/kbd raw scancode device for game input (DOOM)

Added raw scancode ring buffer to the keyboard driver. The HAL
keyboard layer now fires a second callback with the unprocessed
scancode byte (both key-press and key-release events).

- hal/keyboard.h: added hal_keyboard_scan_cb_t and setter
- hal/x86/keyboard.c: fires g_scan_cb before ASCII translation
- drivers/keyboard.c: raw scancode buffer + /dev/kbd device node
  registered via devfs (non-blocking read returns raw scancodes)
- init.c: calls keyboard_register_devfs() after devfs is mounted

DOOM can now open /dev/kbd and read raw PS/2 scancodes to detect
key press/release events without TTY line buffering.