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/Bin.cs

@@ -7,14 +7,36 @@ namespace SawCut
 {
     public class Bin
     {
-        public List<BinItem> Items;
+        private readonly List<BinItem> _items;
 
         public Bin(double length)
         {
-            Items = new List<BinItem>();
+            _items = new List<BinItem>();
             Length = length;
         }
 
+        public IReadOnlyList<BinItem> Items => _items;
+
+        public void AddItem(BinItem item)
+        {
+            _items.Add(item);
+        }
+
+        public void AddItems(IEnumerable<BinItem> items)
+        {
+            _items.AddRange(items);
+        }
+
+        public void RemoveItem(BinItem item)
+        {
+            _items.Remove(item);
+        }
+
+        public void SortItems(Comparison<BinItem> comparison)
+        {
+            _items.Sort(comparison);
+        }
+
         public double Spacing { get; set; }
 
         public double Length { get; set; }