box3d: reuse per-step solver and broad-phase scratch allocations

Persist solver constraint arrays/spans/stage blocks and broad-phase pair
query buffers across steps instead of reallocating each world_step.
Bit-identical results (determinism hash unchanged); washer -7.6%, small
wins on trees/rain, pairs stage -6% on junkyard. A contact-manifold
reuse attempt regressed pyramid scenes and was dropped.

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