feat(emblem): /emblem/update_emblem sets viewer.SelectedEmblem (ownership-validated)

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Emblem/EmblemUpdateRequest.cs

@@ -0,0 +1,13 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Emblem;
+
+[MessagePackObject]
+public class EmblemUpdateRequest : BaseRequest
+{
+    // Spec note: emblem_id is a long on the wire (UserInfo.SelectedEmblemId is also long).
+    [JsonPropertyName("emblem_id")]
+    [Key("emblem_id")]
+    public long EmblemId { get; set; }
+}