Replace simplified BSP overlap test with retail-faithful 6-path
collision dispatcher. Sphere-intersects-poly now uses movement
vector for front-face culling (prevents wall penetration).
All paths: placement/ethereal, checkWalkable, stepDown, collide,
contact+onWalkable, and default (not in contact).
Ported from ACE BSPTree.cs/BSPNode.cs/BSPLeaf.cs/Polygon.cs,
cross-referenced against decompiled chunk_00530000.c.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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