fix(engine): zone should not kill bots before activation

The executeZone function was killing bots outside the zone radius
before the zone became active (ZoneStartTurn). This caused bots to
die at turn 49 when the zone shouldn't start until turn 50.

Added early return when ZoneActive is false, so zone killing only
occurs after the zone has activated.

engine/turn.go

@@ -133,7 +133,11 @@ func (gs *GameState) executeZone() {
 		}
 	}
 
-	// Kill bots outside the zone
+	// Kill bots outside the zone (only when zone is active)
+	if !gs.ZoneActive {
+		return
+	}
+
 	for _, b := range gs.Bots {
 		if !b.Alive {
 			continue