Fix json error.

internal/game/battle.go

@@ -82,8 +82,10 @@ type BattleEvent struct {
 	// "eat":    Seat's pet ate apples; Bonus is its new total.
 	// "heal":   Seat's pet healed; DamageAfter is its new damage total.
 	Type string `json:"type"`
-	Seat   int    `json:"seat,omitempty"`
-	Target int    `json:"target,omitempty"`
+	// Seat/Target must NOT be omitempty: 0 is a valid seat (the first
+	// player) and dropping it makes the client read sides[undefined].
+	Seat   int   `json:"seat"`
+	Target int   `json:"target"`
 	Card   *Card `json:"card,omitempty"`
 	Count  int   `json:"count,omitempty"`
 	// clash: per-seat damage totals / deaths after the exchange.
@@ -91,10 +93,10 @@ type BattleEvent struct {
 	Died   []bool `json:"died,omitempty"`
 	// rock: dice total rolled; rock/strip/steal: target pet's fate.
 	Roll        int  `json:"roll"`
-	DamageAfter int  `json:"damageAfter,omitempty"`
+	DamageAfter int  `json:"damageAfter"`
 	TargetDied  bool `json:"targetDied,omitempty"`
 	// eat: the pet's power bonus after eating.
-	Bonus int `json:"bonus,omitempty"`
+	Bonus int `json:"bonus"`
 }
 
 // BattleResult is the full, public record of one round's battle. Only what