Enable Terrain3D collision_mode=1 so the sculpted landscape provides real physical ground for player and NPCs. Disable GreyboxGround collision (layer/mask = 0) -- it is no longer the active physical floor.
NpcVisual no longer flattens nav waypoint Y to its current elevation. It now follows 3D path points and uses gravity (30 m/s^2) like the player, settling to terrain surface via is_on_floor() + Terrain3D collision. The flat NavigationMesh_greybox still provides correct lateral guidance; physics handles vertical grounding on slopes.
This fixes the player-walking-through-slopes bug and makes NPC movement follow actual terrain elevation for the village area (gentle slopes, ~2.4m noise amplitude). All 10 scenarios pass.
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41b515f474