feat(bp): /battle_pass/buy — crystal-cost + retroactive premium grants

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/BattlePass/BattlePassAchievedInfoDto.cs

@@ -0,0 +1,17 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.BattlePass;
+
+/// <summary>
+/// achieved_info wrapper inside /battle_pass/buy response
+/// (Wizard/BattlePassBuyTask.cs:37-40).
+/// </summary>
+[MessagePackObject]
+public class BattlePassAchievedInfoDto
+{
+    [JsonPropertyName("battle_pass_reward_list")]
+    [Key("battle_pass_reward_list")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public List<BattlePassReceivedRewardDto> BattlePassRewardList { get; set; } = new();
+}