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#4 Copy-portfolio: diversified master mix instead of single-score ranking

The leaderboard ranks single scores, but the top N can be three correlated
weather bots. Suggest a de-clumped set instead:

- Pure CopyPortfolioBuilder (Application): greedily picks high-scoring masters
  while enforcing a per-category cap and a max pairwise correlation; reports who
  was dropped for correlation vs the category cap.
- GET /api/portfolio/suggest?size=8&maxPerCategory=2&maxSimilarity=0.6 builds the
  position-overlap similarity (signed market sets) among copy-relevant masters
  and runs the diversifier. New PortfolioEndpoints group.
- UI: a "Copy-Portfolio" nav page — the diversified picks with category, score,
  copyability, PnL and the pick reason, plus a funnel summary (candidates /
  dropped for correlation / dropped for category cap).
- Tests: top-score order, category cap, correlation drop, size limit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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src/Predictalytics.Application.Tests/Services/CopyPortfolioBuilderTests.cs

@@ -0,0 +1,68 @@
+using System.Collections.Generic;
+using System.Linq;
+using Predictalytics.Application.Services;
+using Xunit;
+
+namespace Predictalytics.Application.Tests.Services;
+
+public class CopyPortfolioBuilderTests
+{
+    private static readonly System.Func<int, int, double> NoCorrelation = (_, _) => 0.0;
+
+    [Fact]
+    public void PicksTopScoresFirst_WhenNoConstraintsBind()
+    {
+        var candidates = new List<PortfolioCandidate>
+        {
+            new(1, 90m, "Politics"),
+            new(2, 80m, "Sports"),
+            new(3, 70m, "Crypto"),
+        };
+        var r = CopyPortfolioBuilder.Build(candidates, NoCorrelation, size: 8, maxPerCategory: 2);
+        Assert.Equal(new[] { 1, 2, 3 }, r.Picks.Select(p => p.TraderId).ToArray());
+    }
+
+    [Fact]
+    public void EnforcesCategoryCap()
+    {
+        // Three Politics masters, cap 2 -> the third (lowest) is dropped for the cap.
+        var candidates = new List<PortfolioCandidate>
+        {
+            new(1, 90m, "Politics"),
+            new(2, 85m, "Politics"),
+            new(3, 80m, "Politics"),
+            new(4, 70m, "Sports"),
+        };
+        var r = CopyPortfolioBuilder.Build(candidates, NoCorrelation, size: 8, maxPerCategory: 2);
+
+        Assert.Equal(new[] { 1, 2, 4 }, r.Picks.Select(p => p.TraderId).ToArray());
+        Assert.Contains(3, r.DroppedForCategoryCap);
+    }
+
+    [Fact]
+    public void DropsHighlyCorrelatedCandidate()
+    {
+        var candidates = new List<PortfolioCandidate>
+        {
+            new(1, 90m, "Politics"),
+            new(2, 85m, "Sports"),   // highly correlated with #1 -> dropped
+            new(3, 80m, "Crypto"),
+        };
+        // 1 and 2 move together; everyone else independent.
+        double Sim(int a, int b) => (a == 1 && b == 2) || (a == 2 && b == 1) ? 0.9 : 0.0;
+
+        var r = CopyPortfolioBuilder.Build(candidates, Sim, size: 8, maxPerCategory: 2, maxSimilarity: 0.6);
+
+        Assert.Equal(new[] { 1, 3 }, r.Picks.Select(p => p.TraderId).ToArray());
+        Assert.Contains(2, r.DroppedForCorrelation);
+    }
+
+    [Fact]
+    public void RespectsSizeLimit()
+    {
+        var candidates = Enumerable.Range(1, 10)
+            .Select(i => new PortfolioCandidate(i, 100m - i, $"Cat{i}")).ToList();
+        var r = CopyPortfolioBuilder.Build(candidates, NoCorrelation, size: 3, maxPerCategory: 2);
+        Assert.Equal(3, r.Picks.Count);
+    }
+}