Example simpler character

[RossNordby]

Asked claude to write a constraint-free character controller. It did so, and the implementation looks reasonable, so this can be treated as some temporary-ish documentation.

Some weaknesses:

1. This simplifies away some of the constraint math; it isn't trying to handle the support's inertia accurately.
2. The character slides down slopes by default because of integration order. This is actually annoying to fix without using constraints given how the solver handles velocity integration right now.
3. It's still fairly complicated given that it's basically an exact mirror of the main character's structure.

I'd like to swing through and do a proper version later once I fiddle with the solver a bit more, but this is better than nothing.