Improved missile guidance

It turns out that primitive Proportional Navigation can't be implemented in space the same way as in the air - the missile needs to reduce its sideways velocity under its own power for it to work.

The missiles now do this, and spawn from a wider range of angles, and now missiles coming from the side seem to naturally avoid the exhaust plume on their approach rather than swinging around and into it as before.

I'd previously discovered some control parameters that would make some missiles overtake the ship before homing in with laser precision (which added a certain element of suspense) but unfortunately I overwrote them during my previous attempts to balance the missiles' accuracy.  The updated control system lacks some flair comparatively but is more consistently challenging.