Interactive example client

[philpax]

Fixes #39. Implements a single-file interactive client for the World Engine with the following properties:

• Supports both Waypoint-1 and Waypoint-1.5 models without issue.
• Correctly implements frame pacing, including a framerate cap. (This was a real challenge! Getting it to behave correctly in all cases - model too fast, model too slow, with CPU overhead, etc - required a lot of iteration.)
• Is well-factored, with state isolated to make it easier to see how to wire things up.
• Works with just the one uv run within this repository.
• Shows both latent and perceptual FPS.
• Implements a pause screen.
• Involves no async or threading.
• Reasonably well-documented. Not sure it needs more, but may have a look over with fresh eyes tomorrow.

Unlike local_world, however, it does not implement seed selection. That should be relatively easy for anyone to add; this is intended to demonstrate the bare minimum of what you need to run our WMs in a game-like environment.

[lapp0]

Code looks reasonable to me. Nice work!

I'll try to test this after our upcoming model freeze.