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.Engine/BestFit/BestFitFilter.cs

@@ -7,7 +7,14 @@ namespace OpenNest.Engine.BestFit
         public double MaxPlateWidth { get; set; }
         public double MaxPlateHeight { get; set; }
         public double MaxAspectRatio { get; set; } = 5.0;
-        public double MinUtilization { get; set; } = 0.3;
+
+        /// <summary>
+        /// A high-aspect pair is kept anyway when this much of its rotated bounding box is
+        /// actual part area. Utilization is only ever an exception here, never a rejection:
+        /// thin-framed, hollow, or concave (e.g. S-shaped) parts have inherently low
+        /// part-to-bbox utilization, yet can nest tightly — their quality is judged by the
+        /// pair bounding-box area the results are sorted on, not by utilization.
+        /// </summary>
         public double UtilizationOverride { get; set; } = 0.75;
 
         public void Apply(List<BestFitResult> results)
@@ -40,16 +47,6 @@ namespace OpenNest.Engine.BestFit
                     continue;
                 }
 
-                if (result.Utilization < MinUtilization)
-                {
-                    result.Keep = false;
-                    result.Reason = string.Format(
-                        "Utilization {0:P0} below minimum",
-                        result.Utilization
-                    );
-                    continue;
-                }
-
                 result.Reason = "Valid";
             }
         }