Newly summoned monsters can intercept an attacker's blow

A drive-by submission, since I was just looking at the Kerkerkruip code, of a minor risk/reward tradeoff.  Under this patch, summoning a monster as a reaction causes the attacker to make a spirit check to hit the defender rather than the summonee; the check's target depends on the summonee's size.  I also clued the mechanic in the Phantasmagoria flavor text.