SPRINGFIELD — For years, the state taxed mobile and manufactured homes as real estate property as if they were built on a permanent foundation.But a policy change in 2011 and an advantageous “grandfather clause” have allowed one couple to slip through the cracks, according to a state appeals court decision.The Mount Vernon-based 5th District Appellate Court ruled this week that a couple should not be taxed at a higher rate under the new law even though they didn’t properly register a home they built …