Sometimes, when you create something and don’t get all dicky when other people want to build on your work, nice things happen. Consider Beyond Black Mesa, an 11-minute fan short set in the world of Valve’s Half-Life 2, using characters from it. You see a lot of this stuff from fans, like the Purchase Brothers’ Escape From City-17, and the still-hopefully-in-development Black Mesa Source. The common thread among all of this is that Valve has …