feat: wire NfpSlideStrategy into BestFitFinder pipeline

Replace RotationSlideStrategy with NfpSlideStrategy in BuildStrategies,
and add integration tests covering the end-to-end FindBestFits pipeline.

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

OpenNest.Tests/NfpBestFitIntegrationTests.cs

@@ -0,0 +1,57 @@
+using OpenNest.CNC;
+using OpenNest.Engine.BestFit;
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class NfpBestFitIntegrationTests
+{
+    [Fact]
+    public void FindBestFits_ReturnsKeptResults_ForSquare()
+    {
+        var finder = new BestFitFinder(120, 60);
+        var drawing = TestHelpers.MakeSquareDrawing();
+        var results = finder.FindBestFits(drawing);
+        Assert.NotEmpty(results);
+        Assert.NotEmpty(results.Where(r => r.Keep));
+    }
+
+    [Fact]
+    public void FindBestFits_ResultsHaveValidDimensions()
+    {
+        var finder = new BestFitFinder(120, 60);
+        var drawing = TestHelpers.MakeSquareDrawing();
+        var results = finder.FindBestFits(drawing);
+
+        foreach (var result in results.Where(r => r.Keep))
+        {
+            Assert.True(result.BoundingWidth > 0);
+            Assert.True(result.BoundingHeight > 0);
+            Assert.True(result.RotatedArea > 0);
+        }
+    }
+
+    [Fact]
+    public void FindBestFits_LShape_HasBetterUtilization_ThanBoundingBox()
+    {
+        var finder = new BestFitFinder(120, 60);
+        var drawing = TestHelpers.MakeLShapeDrawing();
+        var results = finder.FindBestFits(drawing);
+
+        var bestUtilization = results
+            .Where(r => r.Keep)
+            .Max(r => r.Utilization);
+        Assert.True(bestUtilization > 0.5);
+    }
+
+    [Fact]
+    public void FindBestFits_NoOverlaps_InKeptResults()
+    {
+        var finder = new BestFitFinder(120, 60);
+        var drawing = TestHelpers.MakeSquareDrawing();
+        var results = finder.FindBestFits(drawing);
+
+        Assert.All(results.Where(r => r.Keep), r =>
+            Assert.Equal("Valid", r.Reason));
+    }
+}