fix(bestfit): never reject pair candidates by utilization

Thin-framed, hollow, or concave parts (e.g. SULLYS-035's frame) have
inherently low part-to-bbox utilization yet nest tightly, so the 30%
MinUtilization floor wrongly dropped every candidate for them. Pair
quality is judged by the rotated pair bounding-box area the results
are already sorted on; utilization now only ever serves as the
high-aspect exception (UtilizationOverride), never as a rejection.

Adds a hollow-frame helper plus regression tests that kept pairs
exist with low utilization and results stay sorted by pair area.

OpenNest.Tests/BestFit/NfpBestFitIntegrationTests.cs

@@ -42,6 +42,34 @@ public class NfpBestFitIntegrationTests
         Assert.True(bestUtilization > 0.5);
     }
 
+    [Fact]
+    public void FindBestFits_KeepsLowUtilizationFrame_CutoutsDoNotRejectPair()
+    {
+        // A thin-walled hollow frame nests tightly despite tiny part/bbox utilization.
+        // Utilization must never reject a pair; ranking is by pair bounding-box area.
+        var finder = new BestFitFinder(48, 96);
+        var drawing = TestHelpers.MakeFrameDrawing(w: 20, h: 6, t: 0.5);
+
+        var results = finder.FindBestFits(drawing);
+
+        Assert.NotEmpty(results.Where(r => r.Keep));
+        Assert.All(results.Where(r => r.Keep), r => Assert.Equal("Valid", r.Reason));
+        var best = results.Where(r => r.Keep).First();
+        Assert.True(best.Utilization < 0.3);
+    }
+
+    [Fact]
+    public void FindBestFits_ResultsAreSortedByPairBoundingArea()
+    {
+        var finder = new BestFitFinder(48, 96);
+        var results = finder.FindBestFits(TestHelpers.MakeFrameDrawing());
+
+        Assert.True(results.Count > 1);
+        Assert.True(
+            results.Zip(results.Skip(1), (a, b) => a.RotatedArea <= b.RotatedArea).All(x => x)
+        );
+    }
+
     [Fact]
     public void FindBestFits_NoOverlaps_InKeptResults()
     {