fix(guild): populate leader_name in search results (matches prod capture)

Add IViewerRepository.LoadDisplayNamesAsync (batch AsNoTracking select),
extend GuildSearchEntry with LeaderName, enrich GuildService.SearchAsync
with a leader-id batch lookup, and thread the name through the controller's
ToDetailDto call. Adds a leader_name assertion to GuildSearchFlowTests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Repositories/Viewer/ViewerRepository.cs

@@ -286,6 +286,16 @@ public class ViewerRepository : IViewerRepository
         await _dbContext.SaveChangesAsync(ct);
     }
 
+    public async Task<Dictionary<long, string>> LoadDisplayNamesAsync(IReadOnlyCollection<long> viewerIds, CancellationToken ct = default)
+    {
+        if (viewerIds.Count == 0) return new Dictionary<long, string>();
+        return await _dbContext.Set<Models.Viewer>()
+            .AsNoTracking()
+            .Where(v => viewerIds.Contains(v.Id))
+            .Select(v => new { v.Id, v.DisplayName })
+            .ToDictionaryAsync(v => v.Id, v => v.DisplayName, ct);
+    }
+
     private async Task<Models.Viewer> BuildDefaultViewer(string displayName, int initialTutorialState = 1)
     {
         Models.Viewer viewer = new Models.Viewer