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

SVSim.EmulatedEntrypoint/Controllers/EmblemController.cs

@@ -47,4 +47,24 @@ public class EmblemController : SVSimController
         // via the shared Wizard/FavoriteTask.cs Kind enum).
         return new EmptyResponse();
     }
+
+    [HttpPost("update_emblem")]
+    public async Task<ActionResult<EmptyResponse>> Update(EmblemUpdateRequest request, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        var viewer = await _db.Viewers
+            .Include(v => v.Emblems)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+        if (viewer is null) return Unauthorized();
+
+        // EmblemEntry.Id is int; cast the wire long once at the boundary.
+        var owned = viewer.Emblems.FirstOrDefault(e => e.Id == (int)request.EmblemId);
+        if (owned is null)
+            return BadRequest(new { error = "emblem_not_owned" });
+
+        viewer.Info.SelectedEmblem = owned;
+        await _db.SaveChangesAsync(ct);
+        return new EmptyResponse();
+    }
 }

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; }
+}