feat(degree): /degree/update_degree sets viewer.SelectedDegree (ownership-validated)

SVSim.EmulatedEntrypoint/Controllers/DegreeController.cs

@@ -48,4 +48,23 @@ public class DegreeController : SVSimController
         // via the shared Wizard/FavoriteTask.cs Kind enum).
         return new EmptyResponse();
     }
+
+    [HttpPost("update_degree")]
+    public async Task<ActionResult<EmptyResponse>> Update(DegreeUpdateRequest request, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        var viewer = await _db.Viewers
+            .Include(v => v.Degrees)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+        if (viewer is null) return Unauthorized();
+
+        var owned = viewer.Degrees.FirstOrDefault(d => d.Id == request.DegreeId);
+        if (owned is null)
+            return BadRequest(new { error = "degree_not_owned" });
+
+        viewer.Info.SelectedDegree = owned;
+        await _db.SaveChangesAsync(ct);
+        return new EmptyResponse();
+    }
 }

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Degree/DegreeUpdateRequest.cs

@@ -0,0 +1,12 @@
+using MessagePack;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Degree;
+
+[MessagePackObject]
+public class DegreeUpdateRequest : BaseRequest
+{
+    [JsonPropertyName("degree_id")]
+    [Key("degree_id")]
+    public int DegreeId { get; set; }
+}