repo(viewer): restore dropped rationale comment, add link-idempotency assertion

Review polish on the prior commit (30874c6):
- BuildDefaultViewer extract dropped the "filter out Id=0 placeholders
  and dedupe" comment from the leader-skin grant block — restored.
- LinkSteamToViewer test now calls link twice and asserts count stays
  at 1, exercising the alreadyLinked short-circuit.

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

SVSim.Database/Repositories/Viewer/ViewerRepository.cs

@@ -170,6 +170,8 @@ public class ViewerRepository : IViewerRepository
         if (defaultEmblem is not null) viewer.Emblems.Add(defaultEmblem);
         if (defaultBg is not null) viewer.MyPageBackgrounds.Add(defaultBg);
 
+        // Grant one of each class's default leader skin. Filter out the synthetic placeholders
+        // (Id=0) and dedupe — skins are many-to-many via SleeveEntryViewer-style join.
         var grantedSkins = viewer.Classes
             .Select(vcd => vcd.LeaderSkin)
             .Where(s => s.Id != 0)