Collision environment

Collision is URDF-driven. The robot’s covering spheres and self-collision ranges are baked into the generated grid.cuh as GRiD’s grid_collision namespace (pass --collision to scripts/codegen/generate_grid.py); the kernel scores them post-solve via grid_collision::collision_distance (soft penetration cost) and, in hard mode, filters with grid_collision::config_free (see Solver kernel). There is no hand-written per-robot collision header — any URDF gets both FK and collision from one codegen step.

The obstacle set (spheres / cuboids / cylinders from a MotionBenchMaker-style problem JSON) is parsed and uploaded to the device by grid_env.cuh, then passed by value into the scoring kernel as a grid_collision::Environment.