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/Services/IViewerMissionStateService.cs

@@ -0,0 +1,12 @@
+using SVSim.Database.Models;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Lazy-initializes viewer mission/achievement state. Idempotent. Called from
+/// LoadController on every /load/index and as belt-and-braces from /mission/info.
+/// </summary>
+public interface IViewerMissionStateService
+{
+    Task EnsureCurrentAsync(Viewer viewer, CancellationToken ct = default);
+}