Standalone record representing one collision-bearing shape attached to
a logical PhysicsObj. Foundation for the per-part BSP collision fix
that closes the M1.5 "doors don't block" bug. Spec at
docs/superpowers/specs/2026-05-24-door-collision-per-part-bsp-design.md.
No callers in this commit; integration follows in subsequent commits.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
