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/campaign_doc.rs

@@ -145,11 +145,13 @@ pub struct GenDoc {
     pub rivals: u8, // 0..2 rival masters
     #[serde(default = "default_difficulty")]
     pub difficulty: u8, // 0 easy, 1 medium, 2 hard
+    #[serde(default)]
+    pub surface: bool, // the eastern half lies above ground (forest)
 }
 
 impl Default for GenDoc {
     fn default() -> Self {
-        GenDoc { rivers: 1, lava: 0, forts: 1, rivals: 0, difficulty: 1 }
+        GenDoc { rivers: 1, lava: 0, forts: 1, rivals: 0, difficulty: 1, surface: false }
     }
 }