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.UnitTests/Repositories/ViewerRepositoryTests.cs

@@ -155,12 +155,20 @@ public class ViewerRepositoryTests
             await repo.LinkSteamToViewer(viewerId, steamId);
         }
 
+        using (var scope = factory.Services.CreateScope())
+        {
+            var repo = scope.ServiceProvider.GetRequiredService<IViewerRepository>();
+            await repo.LinkSteamToViewer(viewerId, steamId);  // second call must be a no-op
+        }
+
         using var verifyScope = factory.Services.CreateScope();
         var db = verifyScope.ServiceProvider.GetRequiredService<SVSimDbContext>();
         var loaded = await db.Viewers
             .Include(v => v.SocialAccountConnections)
             .FirstAsync(v => v.Id == viewerId);
 
+        // Count == 1 proves both the initial append AND that the second LinkSteamToViewer call
+        // hit the `alreadyLinked` short-circuit (idempotent re-link).
         Assert.That(loaded.SocialAccountConnections, Has.Count.EqualTo(1));
         Assert.That(loaded.SocialAccountConnections[0].AccountType,
             Is.EqualTo(SocialAccountType.Steam));