Improve Document model encapsulation and null safety

- Add backing fields for PartsToNest and StockBins collections
- Ensure collections are never null through property setters
- Add read-only views of collections via new properties
- Improve XML documentation
- Make LastFilePath internally settable for better control

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

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

CutList/Forms/Document.cs

@@ -11,19 +11,50 @@ namespace CutList.Forms
 {
     public class Document
     {
+        private List<PartInputItem> _partsToNest;
+        private List<BinInputItem> _stockBins;
+
         public Document()
         {
-            PartsToNest = new List<PartInputItem>();
+            _partsToNest = new List<PartInputItem>();
-            StockBins = new List<BinInputItem>();
+            _stockBins = new List<BinInputItem>();
         }
 
         [JsonIgnore]
-        public string LastFilePath { get; private set; }
+        public string LastFilePath { get; internal set; }
 
-        public List<PartInputItem> PartsToNest { get; set; }
+        /// <summary>
+        /// Parts to be nested. For JSON serialization, this exposes the list.
+        /// For runtime use, prefer using methods to modify the collection.
+        /// </summary>
+        public List<PartInputItem> PartsToNest
+        {
+            get => _partsToNest;
+            set => _partsToNest = value ?? new List<PartInputItem>();
+        }
 
-        public List<BinInputItem> StockBins { get; set; }
+        /// <summary>
+        /// Stock bins available. For JSON serialization, this exposes the list.
+        /// For runtime use, prefer using methods to modify the collection.
+        /// </summary>
+        public List<BinInputItem> StockBins
+        {
+            get => _stockBins;
+            set => _stockBins = value ?? new List<BinInputItem>();
+        }
 
         public Tool Tool { get; set; }
+
+        /// <summary>
+        /// Gets a read-only view of parts to nest.
+        /// </summary>
+        [JsonIgnore]
+        public IReadOnlyList<PartInputItem> PartsReadOnly => _partsToNest.AsReadOnly();
+
+        /// <summary>
+        /// Gets a read-only view of stock bins.
+        /// </summary>
+        [JsonIgnore]
+        public IReadOnlyList<BinInputItem> StockBinsReadOnly => _stockBins.AsReadOnly();
     }
 }