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.

player/npc/NpcVisual.gd

@@ -185,8 +185,7 @@ func _physics_process(delta: float) -> void:
 		return
 
 	var next_pos := current_path[path_index]
-	var flat_next_pos := Vector3(next_pos.x, global_position.y, next_pos.z)
-	var to_next := flat_next_pos - global_position
+	var to_next := next_pos - global_position
 
 	if to_next.length() <= waypoint_reached_distance:
 		path_index += 1
@@ -197,7 +196,10 @@ func _physics_process(delta: float) -> void:
 
 	velocity.x = direction.x * move_speed
 	velocity.z = direction.z * move_speed
-	velocity.y = 0.0
+	if not is_on_floor():
+		velocity.y -= 30.0 * delta
+	else:
+		velocity.y = 0.0
 
 	move_and_slide()
 	if not global_position.is_equal_approx(prev_pos):