feat(missions): MissionAssembler — single DTO builder reused by all 4 endpoints

Tests intentionally deferred to controller integration tests (Tasks
18-21) which exercise the assembler end-to-end via the wire.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Services/IMissionAssembler.cs

@@ -0,0 +1,13 @@
+using SVSim.Database.Models;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common.Mission;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Builds the MissionInfoDataDto from (viewer, catalog, counters). One place — reused by
+/// all four endpoints. Reads catalog through repos, batches counter reads.
+/// </summary>
+public interface IMissionAssembler
+{
+    Task<MissionInfoDataDto> BuildAsync(Viewer viewer, CancellationToken ct = default);
+}