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/MultiBinEngine.cs

@@ -29,11 +29,11 @@ namespace SawCut.Nesting
                 e.Spacing = Spacing;
                 var r = e.Pack(remainingItems);
 
-                result.Bins.AddRange(r.Bins);
-                remainingItems = r.ItemsNotUsed;
+                result.AddBins(r.Bins);
+                remainingItems = r.ItemsNotUsed.ToList();
             }
 
-            result.ItemsNotUsed = remainingItems;
+            result.AddItemsNotUsed(remainingItems);
 
             return result;
         }