Full engine port in libs/box3d: math, geometry, GJK/TOI, hull builder,
dynamic tree, manifolds, constraint graph, solver (serial, scalar SIMD
path), all 8 joint types, sensors, mover, world API. 147 ported C unit
tests green in debug and release. See libs/box3d/README.md for the
upstream revision and sync notes, PORTING.md for conventions.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
