Auto-deny possible Lazav, Dimir Mastermind transformations if Lazav is no longer in play

For example, in the event of a board wipe with multiple players, Lazav will try to transform into each creature even though Lazav is no longer in play.

The issue names no files, tests, or entry points. Start by reproducing a multiplayer board wipe with Lazav, Dimir Mastermind and multiple possible transformations, then trace the transformation checks. Done means transformations are denied when Lazav is no longer in play, without disrupting valid transformations.

Written by the indexing model from the issue text.