Last week, Realtor.com published another version of its ‘‘magic number’’ forecast. The number in question is the mortgage rate number low enough to ‘‘unfreeze’’ the real estate market.  We know that the market has been in something of a gridlock for over a year now: Home prices are very high and keep rising. Mortgage rates are high and aren’t showing much of a downward trend. There aren’t enough homes to go around, especially those that are remotely affordable. Something has to give.  Learn more about your ad choices. Visit megaphone.fm/adchoices