Pure-computation collision engine for ground-based entity movement.
Three components: TerrainSurface (outdoor heightmap Z interpolation),
CellSurface (indoor EnvCell floor polygon projection), PhysicsEngine
(top-level resolver with step-height enforcement, outdoor/indoor
cell transitions via CellPortals, and gravity reporting).
Uses PhysicsPolygons from CellStruct for walkable surfaces with
brute-force polygon iteration (< 20 polys per cell). BSP tree
acceleration deferred — same collision fidelity, simpler code.
Standalone module with no rendering or networking dependencies.
~15-20 unit tests with fake data covering flat terrain, slopes,
stairs, wall rejection, and cell transitions. Integration with
the streaming system via ApplyLoadedTerrainLocked. Consumed by
Phase B.2 (player movement mode, separate spec).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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