It’s hush-hush, but the blockbuster ratings-grabber twist brewing on “Spin City” is . . . a WEDDING for Michael J. Fox when he returns this fall for a handful of episodes! Here’s the dilly-o: Michael shows up with a new fiancee named Allison — and Heather Locklear’s character goes NUTS with jealousy! Producers are searching for just the right actress to play a bright, drop-dead gorgeous professional in her late 20s who works for the World Wildlife Association! (Hate her already, ladies?) If onscreen chemistry clicks, Michael and Miss Perfect will wed onscreen!