fix(physics): validate retail cell containment roots

tests/AcDream.Core.Tests/Physics/CellTransitFindVisibleChildCellTests.cs

@@ -106,7 +106,9 @@ public class CellTransitFindVisibleChildCellTests
         InverseWorldTransform = Matrix4x4.Identity,
         Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
         CellBSP               = new CellBSPTree { Root = cellBspRoot },
-        Portals               = Array.Empty<PortalInfo>(),
+        // Keep this synthetic cell eligible for retail point_in_cell; the
+        // test varies containment and stab-list behavior, not portal absence.
+        Portals               = [new PortalInfo(0xFFFF, 0, 0)],
         PortalPolygons        = new Dictionary<ushort, ResolvedPolygon>(),
         VisibleCellIds        = new HashSet<uint>(visibleCellIds),
     };