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/BestFitEngine.cs

@@ -23,16 +23,18 @@ namespace SawCut.Nesting
             Items = items.OrderByDescending(i => i.Length).ToList();
 
             var result = new Result();
-            result.ItemsNotUsed = Items.Where(i => i.Length > StockLength).ToList();
+            var itemsTooLarge = Items.Where(i => i.Length > StockLength).ToList();
+            result.AddItemsNotUsed(itemsTooLarge);
 
-            foreach (var item in result.ItemsNotUsed)
+            foreach (var item in itemsTooLarge)
             {
                 Items.Remove(item);
             }
 
-            result.Bins = GetBins();
+            var bins = GetBins();
+            result.AddBins(bins);
 
-            foreach (var bin in result.Bins)
+            foreach (var bin in bins)
             {
                 foreach (var item in bin.Items)
                 {
@@ -40,7 +42,7 @@ namespace SawCut.Nesting
                 }
             }
 
-            result.ItemsNotUsed.AddRange(Items);
+            result.AddItemsNotUsed(Items);
 
             return result;
         }
@@ -66,7 +68,7 @@ namespace SawCut.Nesting
                 }
 
                 if (best_bin != null)
-                    best_bin.Items.Add(item);
+                    best_bin.AddItem(item);
 
                 
             }