fix: ground physics on real Terrain3D surface

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.

world/jajce/JajceWorld.tscn

@@ -121,6 +121,8 @@ script = ExtResource("3_world")
 navigation_mesh = SubResource("NavigationMesh_greybox")
 
 [node name="GreyboxGround" type="StaticBody3D" parent="NavigationRegion3D"]
+collision_layer = 0
+collision_mask = 0
 
 [node name="Mesh" type="MeshInstance3D" parent="NavigationRegion3D/GreyboxGround"]
 visible = false
@@ -136,7 +138,7 @@ data_directory = "res://terrain/jajce/data"
 material = SubResource("Terrain3DMaterial_jajce")
 assets = ExtResource("1_assets")
 collision_mask = 3
-collision_mode = 0
+collision_mode = 1
 top_level = true
 
 [node name="FortressBlockout" type="Node3D" parent="."]