Final missions above ground (M30, co-op M20), surface option for versus matches, docs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/surface.rs

@@ -183,4 +183,16 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn the_finale_is_fought_above_ground() {
+        let g = crate::campaign::start_mission(29, None);
+        assert!(g.surface_share() > 0.3, "share {}", g.surface_share());
+        assert_eq!(g.rivals.len(), 2);
+        assert!(g.gate_hp > 0.0 && g.has_gate());
+        let (gx, gy) = g.map.gate;
+        assert!(g.map.sky(idx(gx + 1, gy + 1)), "the gate of the order stands in the forest");
+        // the heroes find their way from the gate to the core
+        assert!(g.map.hero_path_to_core(idx(gx + 1, gy + 1)).is_some());
+    }
 }