To be specific: there is no "full" version of PS. The thing you downloaded is the most complete and recent release you can get (without compiling the CVS). It's called "Tech demo" because it isn't by far complete enough to classify as game yet, hence it more or less just demonstrates the technology used to make it. However, it can already be played, but you'll find lots of things missing or not yet / anymore working. It says this to avoid raising high expectations, since before it did so, we got a lot of people complaining that the game wasn't playable.