fix: small parts use FindScrapZones not FindAllRemnants

Small parts must only go into scrap zones (both dims < minRemnantSize)
to preserve viable remnants. The implementer had inverted this, giving
small parts access to all remnants. Also fixed the test to verify
remnant preservation behavior and removed unused FindAllRemnants helper.

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

OpenNest.Engine/MultiPlateNester.cs

@@ -262,9 +262,9 @@ namespace OpenNest
                     if (classification == PartClass.Large)
                         continue; // Large parts don't go on existing plates — they create new ones.
 
-                    // Get all remnants and try to place in them.
+                    // Small parts only go into scrap zones; medium parts into viable remnants.
                     var remnants = classification == PartClass.Small
-                        ? FindAllRemnants(pr.Plate)
+                        ? FindScrapZones(pr.Plate, minRemnantSize)
                         : FindViableRemnants(pr.Plate, minRemnantSize);
 
                     foreach (var zone in remnants)
@@ -435,12 +435,6 @@ namespace OpenNest
             return false;
         }
 
-        private static List<Box> FindAllRemnants(Plate plate)
-        {
-            var finder = RemnantFinder.FromPlate(plate);
-            return finder.FindRemnants();
-        }
-
         private static NestItem CloneItem(NestItem item)
         {
             return new NestItem