fix(physics): restore retail cell availability semantics

tests/AcDream.Core.Tests/World/Cells/EnvCellTests.cs

@@ -27,6 +27,23 @@ public class EnvCellTests
     public void PointInCell_NullBsp_Aabb_OutsideIsFalse()
         => Assert.False(Make(new Vector3(0,0,0), new Vector3(10,10,10)).PointInCell(new Vector3(20,5,5)));
 
+    [Fact]
+    public void PointInCell_LoadedNullRoot_IsUniversallyInside()
+    {
+        var cell = new EnvCell(
+            0xA9B4_0174u,
+            Matrix4x4.Identity,
+            Matrix4x4.Identity,
+            Vector3.Zero,
+            Vector3.One,
+            Array.Empty<UcgCellPortal>(),
+            Array.Empty<uint>(),
+            seenOutside: false,
+            containmentBsp: new CellBSPTree { Root = null });
+
+        Assert.True(cell.PointInCell(new Vector3(10_000f, -10_000f, 500f)));
+    }
+
     [Fact]
     public void PointInCell_TransformsWorldToLocalBeforeTesting()
     {