feat: /dev/fb0 framebuffer device + fd-backed mmap support

feat: /dev/fb0 framebuffer device + fd-backed mmap support

Added /dev/fb0 device node registered via devfs by vbe.c:
- ioctl: FBIOGET_VSCREENINFO (resolution, bpp), FBIOGET_FSCREENINFO
  (phys addr, pitch, size)
- mmap: maps physical framebuffer into userspace with NOCACHE flags
- read/write: direct pixel buffer access via offset

Extended syscall_mmap_impl to support fd-backed mmap: when
MAP_ANONYMOUS is not set, the file descriptor's node->mmap callback
is invoked. This enables userspace to mmap /dev/fb0 for direct
framebuffer access (required for DOOM).

Marked syscall_mmap_impl as noinline to prevent GCC from merging it
into syscall_handler (4KB kernel stack limit).