content: Don't have enclosure/padding around underground exhibits.

all-is-cubes-content/src/city.rs

@@ -361,7 +361,12 @@ async fn place_exhibits_in_city<I: Instant>(
 
         // Amount by which the exhibit's own size is expanded to form walls and empty space
         // for displaying it and allowing players to move around it.
-        let enclosure_thickness = FaceMap::repeat(1);
+        let enclosure_thickness = match exhibit.placement {
+            Placement::Surface => FaceMap::repeat(1),
+            // Underground exhibits get no enclosure; they are expected to play nicely with being
+            // buried in stone except for the entranceway.
+            Placement::Underground => FaceMap::repeat(0),
+        };
 
         // Now that we know the size of the exhibit, find a place for it that fits its bounds.
         let exhibit_footprint = exhibit_space.bounds();