Retail's CPartArray::InitParts emits collision shapes only from
Setup-level CylSpheres/Spheres or per-Part PhysicsBSP — never
from visual mesh AABBs. The predicate captures the retail rule:
a stab whose source is a GfxObj (high byte 0x01) with no cached
GfxObjPhysics is phantom (no collision). Wired into GameWindow's
mesh-aabb-fallback synthesis in the next commit.
Refs docs/research/2026-05-25-a6-stairs-cyl-retail-investigation.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
