refactor: load/save state as ptr

internal/model/game/battle.go

@@ -15,7 +15,7 @@ type Battle struct {
 	BattleReport BattleReport
 
 	shipAmmo map[int]uint
-	attacker map[int]map[int]float64 // a group able to attack and destroy an opponent with some probability
+	attacker map[int]map[int]float64 // a group able to attack and destroy an opponent with some probability > 0
 }
 
 type BattleReport struct {
@@ -200,7 +200,7 @@ func SingleBattle(g *Game, b *Battle) {
 			case probability > 0:
 				destroyed = rand.Float64() >= probability
 			default:
-				panic("probabilities cache returned unexpected value")
+				panic("SingleBattle: probability unexpected: value <= 0")
 			}
 
 			b.BattleReport.BattleAction = append(b.BattleReport.BattleAction, BattleAction{