fix(physics): restore retail cell availability semantics

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

@@ -160,4 +160,50 @@ public sealed class PhysicsDataCacheProductionTests
             structure.ContainmentBsp,
             runtimeCell.FlatContainmentBsp);
     }
+
+    [Fact]
+    public void ProductionCellPublication_PreservesLoadedCellWithEmptyRoots()
+    {
+        const uint cellId = 0xA9B4_0174u;
+        PhysicsDataCache cache = PhysicsDataCache.CreateProduction();
+        var emptyPhysics = new FlatPhysicsBsp(
+            -1,
+            ImmutableArray<FlatPhysicsBspNode>.Empty,
+            ImmutableArray<int>.Empty,
+            FlatPolygonTable.Empty);
+        var emptyContainment = new FlatCellContainmentBsp(
+            -1,
+            ImmutableArray<FlatCellBspNode>.Empty);
+        var structure = new FlatCellStructureCollisionAsset(
+            emptyPhysics,
+            emptyContainment,
+            FlatPolygonTable.Empty);
+        var topology = new FlatEnvCellTopology(
+            ImmutableArray<FlatEnvCellPortal>.Empty,
+            ImmutableArray<uint>.Empty,
+            seenOutside: false);
+
+        cache.CacheCellStruct(
+            cellId,
+            new EnvCell(),
+            Matrix4x4.Identity,
+            structure,
+            topology);
+
+        CellPhysics loaded = Assert.IsType<CellPhysics>(
+            cache.GetCellStruct(cellId));
+        Assert.False(CollisionTraversal.HasPhysics(cache, loaded));
+        Assert.True(CollisionTraversal.HasCellContainment(cache, loaded));
+        Assert.True(CollisionTraversal.PointInsideCell(
+            cache,
+            loaded,
+            new Vector3(10_000f, -10_000f, 500f)));
+        var graphCell = Assert.IsType<AcDream.Core.World.Cells.EnvCell>(
+            cache.CellGraph.GetVisible(cellId));
+        Assert.True(graphCell.PointInCell(
+            new Vector3(10_000f, -10_000f, 500f)));
+        Assert.Equal(1, cache.CellStructCount);
+        Assert.Equal(1, cache.FlatCellStructCount);
+        Assert.Equal(0, cache.GraphCellStructCount);
+    }
 }