refactor: clean up MultiPlateNester code smells and duplication

Extract shared patterns into reusable helpers: FitsBounds (fits-normal/
rotated check), OptionWorkArea (edge-spacing subtraction), DecrementQuantity,
TryWithUpgradedSize (upgrade-try-revert), FindSmallestFittingOption.
Add PlateResult.AddParts to consolidate dual parts-list bookkeeping.
Cache sorted plate options and add HasPlateOptions property. Introduce
MultiPlateNestOptions to replace 10-parameter Nest signature with a
clean options object. Fix fragile Drawing.Name matching with reference
equality in PackIntoExistingRemnants.

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

OpenNest.Engine/MultiPlateResult.cs

@@ -2,6 +2,16 @@ using System.Collections.Generic;
 
 namespace OpenNest
 {
+    public class MultiPlateNestOptions
+    {
+        public Plate Template { get; set; }
+        public List<PlateOption> PlateOptions { get; set; }
+        public double SalvageRate { get; set; } = 0.5;
+        public PartSortOrder SortOrder { get; set; } = PartSortOrder.BoundingBoxArea;
+        public double MinRemnantSize { get; set; } = 12.0;
+        public bool AllowPlateCreation { get; set; } = true;
+    }
+
     public class MultiPlateResult
     {
         public List<PlateResult> Plates { get; set; } = new();
@@ -14,5 +24,11 @@ namespace OpenNest
         public List<Part> Parts { get; set; } = new();
         public PlateOption ChosenSize { get; set; }
         public bool IsNew { get; set; }
+
+        public void AddParts(IList<Part> parts)
+        {
+            Plate.Parts.AddRange(parts);
+            Parts.AddRange(parts);
+        }
     }
 }