fix #451: stabilize portal seam rendering

src/AcDream.App/Rendering/WorldRenderFrameBuilder.cs

@@ -43,6 +43,21 @@ internal readonly record struct WorldRootFrame(
     bool PlayerIndoorGate)
 {
     public bool RenderSky => ViewerRoot is null || RootSeenOutside;
+
+    /// <summary>
+    /// The camera has an EnvCell root whose authored visibility does not reach
+    /// outdoors. SeenOutside cells are open-air presentation cells, not an
+    /// indoor environment gate merely because they have portal geometry.
+    /// </summary>
+    public bool CameraInsideEnclosedCell => CameraInsideCell && !RootSeenOutside;
+
+    /// <summary>Environment gate shared by directional shadows and other
+    /// outdoor-only render-pack effects.</summary>
+    public bool PlayerOrCameraInsideEnclosedCell =>
+        PlayerInsideCell || CameraInsideEnclosedCell;
+
+    public bool IsAtmosphericallyOutdoor =>
+        RenderSky && !CameraInsideEnclosedCell;
 }
 
 /// <summary>Borrowed building scratch, valid only until the next build.</summary>