Emit a warning when agents move without updating their velocity

This just bit me in the ass, as my character slowed down when taking curves. Turns out my system for setting its velocity never ran, so landmass was working off of invalid data. Would be nice if we e.g. used change detection on `Transform` and on the agent velocity to ensure they're updated in tandem. Preferably add that warning as an options resource so users can toggle it on and off at will.