test: add RemnantFinder edge cases and FillScore comparison tests

RemnantFinder: obstacle clipping, overlapping obstacles, iterative
workflow, grid pattern, no-overlap invariant, constructor/AddObstacles.
FillScore: count-vs-density ordering, operators, Compute edge cases.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/FillScoreTests.cs

@@ -0,0 +1,89 @@
+namespace OpenNest.Tests;
+
+public class FillScoreTests
+{
+    [Fact]
+    public void HigherCount_WinsOverLowerCount()
+    {
+        var a = new FillScore(10, 0.5);
+        var b = new FillScore(5, 0.9);
+
+        Assert.True(a > b);
+        Assert.False(b > a);
+    }
+
+    [Fact]
+    public void SameCount_HigherDensityWins()
+    {
+        var a = new FillScore(10, 0.8);
+        var b = new FillScore(10, 0.5);
+
+        Assert.True(a > b);
+        Assert.False(b > a);
+    }
+
+    [Fact]
+    public void EqualScores_AreNotGreaterOrLess()
+    {
+        var a = new FillScore(10, 0.5);
+        var b = new FillScore(10, 0.5);
+
+        Assert.False(a > b);
+        Assert.False(a < b);
+        Assert.True(a >= b);
+        Assert.True(a <= b);
+    }
+
+    [Fact]
+    public void Default_IsZero()
+    {
+        var score = default(FillScore);
+
+        Assert.Equal(0, score.Count);
+        Assert.Equal(0, score.Density);
+    }
+
+    [Fact]
+    public void Compute_NullParts_ReturnsDefault()
+    {
+        var score = FillScore.Compute(null, new Geometry.Box(0, 0, 100, 100));
+
+        Assert.Equal(0, score.Count);
+    }
+
+    [Fact]
+    public void Compute_EmptyParts_ReturnsDefault()
+    {
+        var score = FillScore.Compute(new System.Collections.Generic.List<Part>(), new Geometry.Box(0, 0, 100, 100));
+
+        Assert.Equal(0, score.Count);
+    }
+
+    [Fact]
+    public void Compute_WithParts_ReturnsCorrectCount()
+    {
+        var parts = new System.Collections.Generic.List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 10),
+            TestHelpers.MakePartAt(20, 0, 10),
+            TestHelpers.MakePartAt(40, 0, 10)
+        };
+        var score = FillScore.Compute(parts, new Geometry.Box(0, 0, 100, 100));
+
+        Assert.Equal(3, score.Count);
+        Assert.True(score.Density > 0);
+    }
+
+    [Fact]
+    public void CompareTo_IsConsistentWithOperators()
+    {
+        var a = new FillScore(10, 0.8);
+        var b = new FillScore(5, 0.9);
+
+        Assert.True(a.CompareTo(b) > 0);
+        Assert.True(a > b);
+        Assert.True(b < a);
+        Assert.True(a >= b);
+        Assert.True(b <= a);
+    }
+}