box3d: rapier comparison — ~2x faster single-threaded, bench in libs/rapier/crates/bench

Same scenes/geometry/materials/dt, matched solver budget (4 substeps vs
4 solver iterations), interleaved min-of-4 runs: large_pyramid 2.23x,
many_pyramids 1.82x, joint_grid 1.88x (geomean ~1.97x). Table + fairness
notes at the top of the box3d README (vendored rapier has no SIMD;
enhanced-determinism measured free on these scenes).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>