feat: add IFillComparer interface and DefaultFillComparer

Extracts the fill result scoring contract into IFillComparer with a DefaultFillComparer implementation that preserves the existing count-then-density lexicographic ranking via FillScore.

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

OpenNest.Tests/FillComparerTests.cs

@@ -0,0 +1,65 @@
+using OpenNest.Engine;
+using OpenNest.Engine.Fill;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class DefaultFillComparerTests
+{
+    private readonly IFillComparer comparer = new DefaultFillComparer();
+    private readonly Box workArea = new(0, 0, 100, 100);
+
+    [Fact]
+    public void NullCandidate_ReturnsFalse()
+    {
+        var current = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        Assert.False(comparer.IsBetter(null, current, workArea));
+    }
+
+    [Fact]
+    public void EmptyCandidate_ReturnsFalse()
+    {
+        var current = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        Assert.False(comparer.IsBetter(new List<Part>(), current, workArea));
+    }
+
+    [Fact]
+    public void NullCurrent_ReturnsTrue()
+    {
+        var candidate = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        Assert.True(comparer.IsBetter(candidate, null, workArea));
+    }
+
+    [Fact]
+    public void HigherCount_Wins()
+    {
+        var candidate = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 10),
+            TestHelpers.MakePartAt(20, 0, 10),
+            TestHelpers.MakePartAt(40, 0, 10)
+        };
+        var current = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 10),
+            TestHelpers.MakePartAt(20, 0, 10)
+        };
+        Assert.True(comparer.IsBetter(candidate, current, workArea));
+    }
+
+    [Fact]
+    public void SameCount_HigherDensityWins()
+    {
+        var candidate = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 10),
+            TestHelpers.MakePartAt(12, 0, 10)
+        };
+        var current = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 10),
+            TestHelpers.MakePartAt(50, 0, 10)
+        };
+        Assert.True(comparer.IsBetter(candidate, current, workArea));
+    }
+}