fix portal placement

Assets/Engine/Runtime/Requirements/Placement/PlaceOnBottomRow.cs

@@ -0,0 +1,24 @@
+using System.Collections.Generic;
+using EscapeRoomEngine.Engine.Runtime.Modules;
+using UnityEngine;
+
+namespace EscapeRoomEngine.Engine.Runtime.Requirements.Placement
+{
+    /// <summary>
+    /// This requirement guarantees that the back side of the module is placed on the bottom row of the space.
+    /// </summary>
+    [CreateAssetMenu(menuName = "Requirements/Place On Bottom Row")]
+    public class PlaceOnBottomRow : PlacementRequirement
+    {
+        protected override List<Utilities.Placement> FilterCandidates(List<Utilities.Placement> candidates, Module module, Space space)
+        {
+            candidates.RemoveAll(candidate =>
+            {
+                var (left, right) = candidate.BackCorners;
+                return !(left.z == 0 && right.z == 0);
+            });
+
+            return candidates;
+        }
+    }
+}