Physics playground

1. Install rerun and cargo watch

   cargo install cargo-watch cargo install -f rerun-cli

2. Start viewer in one terminal:

   rerun

3. Watch for changes and make a new recording when saving the source code:

   cargo watch -x 'run --release --bin xpbd_particles'

or do this to run clippy first:

cargo watch -x 'clippy --release' -x 'run --release --bin xpbd_particles'

4. Click play or loop in the viewer to see the simulation.