Live capture #3 (cp-write + entity-source + full bsp-test object map) disproved
v1's grounding-retention theory and pinned the real bug: GameWindow.cs:7951
partId = entity.Id*256+partIndex OVERFLOWS uint32 for class-prefixed landblock
ids (0x40/0x80/0xC0), dropping the prefix byte so different-class entities sharing
the low 24 bits collide on one shadow part-id; Register deregisters the loser
(last-writer-wins), silently deleting collision geometry while render shows every
step. Landblock 0xF682 has 23 such collisions incl. the stair runs. The player
floats into the collision hole and the PrecipiceSlide wedge fires = the invisible
wall (a faithful symptom). Fix = Option A: RegisterMultiPart per entity (unique
32-bit entity.Id, retail add_shadows_to_cells/AddPartsShadow model), unifying on
the one faithful multi-part path and deleting both synthetic-id schemes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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