Simplified spatial lookup implementation

[nfagerlund]

I was gonna upgrade to bevy 0.10, but as it happens I JUsT picked up a dependency on bevy_spatial, whose author decided to do a rewrite before supporting 0.10. Whoooops

But the code's pretty comprehensible and 90% of its complexity comes from being more generic than I need. So I can just rip out that rstar crate dep on its own and do a chopped-down re-impl.

HOWEVER, it also turns out that bevy_spatial was using (local) Transform, and I need to be using my physics space transforms instead to get global accuracy for objects that participate in physics. And that might add some more complexity.

So, as a pre-requisite for this, it's time to codify the small offset hack I was doing into a more ergonomic interface for handling positions.

[nfagerlund]

OK so uhhhhh let's see. About that.

• I've already got that SubTransform component sitting around from when I was using it to lock Transform to pixel boundaries! Nice!!! I can repurpose that as my PhysicalTransform and just insist movement has to go through that component.
• So, if something has a PhysicalTransform and a PhysicalOffset, we take both into account when syncing PhysicalTransform to Transform at the end of the frame.
• How to initialize? Query for (Added<PhysicalOffset>, Without<PhysicalTransform>), and initialize physical transform from Transform - Offset.
• Should be good to go at that point 🤷

[nfagerlund]

Yeah okay, that worked great! Now onwards to building out my own spatial lookup utilization.

[nfagerlund]

All right, that ALSO worked great.