feat(missions): ViewerMissionStateService — lazy materialize achievements + assign slots

Reads existing state from DB on each call (don't trust navigation
property — caller may pass it stale or double-tracked). Adds via DbSet
only, not via navigation property, to avoid EF double-tracking.

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

SVSim.EmulatedEntrypoint/Program.cs

@@ -93,6 +93,7 @@ public class Program
         builder.Services.AddScoped<SVSim.Database.Repositories.Mission.IViewerMissionRepository,
                                     SVSim.Database.Repositories.Mission.ViewerMissionRepository>();
         builder.Services.AddScoped<IMissionProgressService, MissionProgressService>();
+        builder.Services.AddScoped<IViewerMissionStateService, ViewerMissionStateService>();
         builder.Services.AddSingleton<TimeProvider>(TimeProvider.System);
         builder.Services.AddScoped<IStoryMasterRepository, StoryMasterRepository>();
         builder.Services.AddScoped<IViewerStoryProgressRepository, ViewerStoryProgressRepository>();