Encapsulate mutable collections in Bin and Result

Replace public mutable collection fields/properties with private
backing fields and expose them as IReadOnlyList. Add proper methods
for mutation (AddItem, AddItems, RemoveItem, SortItems).

Changes:
- Bin.Items: Now private with AddItem/AddItems/RemoveItem/SortItems
- Result.ItemsNotUsed: Now readonly with Add methods
- Result.Bins: Now readonly with Add methods
- Updated all engine classes to use new encapsulated APIs

This improves encapsulation and prevents external code from
bypassing business logic by directly manipulating collections.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

SawCut/Nesting/Result.cs

@@ -4,14 +4,37 @@ namespace SawCut.Nesting
 {
     public class Result
     {
+        private readonly List<BinItem> _itemsNotUsed;
+        private readonly List<Bin> _bins;
+
         public Result()
         {
-            ItemsNotUsed = new List<BinItem>();
-            Bins = new List<Bin>();
+            _itemsNotUsed = new List<BinItem>();
+            _bins = new List<Bin>();
         }
 
-        public List<BinItem> ItemsNotUsed { get; set; }
+        public IReadOnlyList<BinItem> ItemsNotUsed => _itemsNotUsed;
 
-        public List<Bin> Bins { get; set; }
+        public IReadOnlyList<Bin> Bins => _bins;
+
+        public void AddItemNotUsed(BinItem item)
+        {
+            _itemsNotUsed.Add(item);
+        }
+
+        public void AddItemsNotUsed(IEnumerable<BinItem> items)
+        {
+            _itemsNotUsed.AddRange(items);
+        }
+
+        public void AddBin(Bin bin)
+        {
+            _bins.Add(bin);
+        }
+
+        public void AddBins(IEnumerable<Bin> bins)
+        {
+            _bins.AddRange(bins);
+        }
     }
 }