IPAddr#ipv4_mapped

# File tmp/rubies/ruby-3.4.1/lib/ipaddr.rb, line 358
def ipv4_mapped
  if !ipv4?
    raise InvalidAddressError, "not an IPv4 address: #{@addr}"
  end
  clone = self.clone.set(@addr | 0xffff00000000, Socket::AF_INET6)
  clone.instance_variable_set(:@mask_addr, @mask_addr | 0xffffffffffffffffffffffff00000000)
  clone
end