Co-op matches of editor campaigns

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

src/game.rs

@@ -1624,9 +1624,12 @@ impl Game {
     }
 
     fn after_economy(&mut self, dt: f32) {
-        // from here on it is about the local master and the humans: that side is switched in first
+        // the shared world (waves, units, towers, rivals ...) runs from the first master's side: the
+        // same on every computer of a match, whichever master plays at this one. Single-player
+        // games only have that one side.
         let local = self.local_side;
-        self.switch_to(local);
+        let first = self.master_sides().first().copied().unwrap_or(local);
+        self.switch_to(first);
         if self.waves_on() {
             self.wave_tick(dt);
         }
@@ -1636,7 +1639,7 @@ impl Game {
 
         self.update_contested();
         self.update_units(dt);
-        self.switch_to(local); // update_units worked master by master
+        self.switch_to(first); // update_units worked master by master
         self.update_towers(dt);
         self.update_reveals(dt);
         self.update_graves(dt);
@@ -1644,6 +1647,8 @@ impl Game {
             self.update_rivals(dt);
         }
         self.update_dwarves(dt);
+        // the end of the game and the fog are about the master at this computer
+        self.switch_to(local);
         self.check_end();
         self.update_fog(dt);
     }