Surface world: grass in daylight, forests, cliffs with cave mouths; free-game and editor option

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

src/settings.rs

@@ -162,6 +162,7 @@ pub struct Settings {
     pub difficulty: u8, // rival masters in free games: 0 easy, 1 medium, 2 hard
     pub map_w: usize, // size of new maps (64..256)
     pub map_h: usize,
+    pub surface: bool, // new free games: the eastern half lies above ground (forest)
     // key bindings: action id -> key name
     pub keys: BTreeMap<String, String>,
 }
@@ -196,6 +197,7 @@ impl Default for Settings {
             difficulty: 1,
             map_w: 64,
             map_h: 64,
+            surface: false,
             keys: BTreeMap::new(),
         }
     }