Experiment with reaction convergence

[viridia]

Sometimes reactions trigger other reactions, especially in the case of derived signals. However, since we're using Bevy change detection, this will cause a 1-frame delay: when a reaction causes a mutation, we won't see the change until the next frame.

We can avoid this by putting the reactive logic in a separate schedule and running that schedule in a loop. This will need to check for convergence, however: the number of changes should decrease to zero in just a few iterations. We can count how many dependencies change each iteration, and insure that that number generally decreases - it may increase sometimes, but not too often.

Quill had a similar algorithm that used atomic ints, but it should be possible to get this to work with Bevy changes.