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/campaign.rs

@@ -358,16 +358,19 @@ pub fn missions() -> Vec<MissionDef> {
             .boss(Paladin, 10)],
         ..base("m20", (160, 160), 2020, vec![Goal::Heart, Goal::Shards])
     });
+    // the finale above ground: the dungeon in the rock, the eastern half a forest with the High
+    // Marshal's fortress (the gate of the order) and both rival masters
     v.push(MissionDef {
         portals: 3,
         gold: 6000.0,
-        gate: true,
         waves: true,
         first_wave: 420.0,
         gate_defenders: true,
-        rivers: vec![river(false, true, 0.45, 4, 2), river(true, false, 0.5, 4, 2)],
-        islands: vec![island((0.3, 0.2), false, true, 3000)],
-        rivals: vec![(0.85, 0.22, 2, 2), (0.85, 0.78, 2, 3)],
+        rivers: vec![river(false, true, 0.3, 4, 2)],
+        islands: vec![island((0.2, 0.2), false, true, 3000)],
+        forts: vec![fort((0.8, 0.5), (27, 25), &[3], &[Arrow, Cannon, Magic, Arrow, Cannon, Magic, Arrow, Cannon], 3, 14, 4, 4, 3, 3000).heart().boss(Paladin, 10)],
+        rivals: vec![(0.86, 0.15, 2, 2), (0.86, 0.85, 2, 3)],
+        surface: 0.48,
         ..base("m30", (176, 176), 3030, vec![Goal::Rivals, Goal::Heart])
     });
     v