dto: SignupRequest + empty SignupResponse

Request mirrors LoginPostParams (device telemetry); response is empty
because all signup outputs live in data_headers (viewer_id, short_udid,
udid). MessagePackObject + Key mirrors JsonPropertyName per project
convention.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Responses/SignupResponse.cs

@@ -0,0 +1,10 @@
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses;
+
+/// <summary>
+/// <c>POST /tool/signup</c> response. The interesting outputs (viewer_id, short_udid, udid) all
+/// live in <c>data_headers</c>; the <c>data</c> payload is empty. <c>SignUpTask.Parse</c> never
+/// reads <c>data</c>.
+/// </summary>
+public class SignupResponse
+{
+}